Patent Publication Number: US-7221895-B2

Title: Image forming apparatus and belt unit therefor, and image forming system

Description:
This application is a Continuation application of Ser. No. 10/321,455 filed on Dec. 18, 2002 now U.S. Pat. No. 6,941,094. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an image forming apparatus that forms a visible image on an image bearing member to obtain a recorded image, a belt unit used in the image forming apparatus, and an image forming system including the image forming apparatus. 
   2. Discussion of the Background 
   Image forming apparatuses in which a recorded image is obtained by forming a visible image on an image bearing member are known, e.g., an electrophotographic copying machine, a printer, a facsimile apparatus, and a multi-function apparatus having at least two functions of copying, printing and facsimile functions. Various types of image forming apparatuses having different functions are known. For example, some image forming apparatuses can form an image of relative large size, others can only form an image of relatively small size. Further, while some image forming apparatuses can form an image on each side of a recording medium, others form an image only on one side of a recording medium. Each user selects and obtains one of these image forming apparatuses having different functions based on its needs. However, it often happens that after obtaining an image forming apparatus having certain functions, the needs of the user change and it becomes necessary to obtain image forming apparatuses having different functions. In this case, the user is forced to bear a great financial burden. 
   An image forming apparatus is known to form a full color image on each side of a recording medium by first transferring visible images of different colors formed on a plurality of image bearing members onto an intermediary transfer member by superimposing one image of different color upon another, thereby forming a first full color image thereupon, transferring the first full color image onto an intermediary image bearing member, then transferring further visible images of different colors subsequently formed on the plurality of image bearing members onto an upper surface of a recording medium being conveyed between the plurality of image bearing members and the intermediary transfer member by superimposing one further image of different color upon another, thereby forming a second full color image on the upper surface of the recording medium, and further, transferring the first full color image on the intermediary image bearing member onto a lower surface of the recording medium. 
   In the above-described image forming apparatus, if an image to be formed is smaller in size than a surface size of the intermediary image bearing member, the image can be formed on each side of the recording medium. Accordingly, if the surface size of the intermediary image bearing member is set to such a size that corresponds to a recording medium of a maximum size that can be used in the image forming apparatus, a color image can be formed on each side of any recording medium used in the image forming apparatus. For example, when the maximum size of a recording medium that can be used in an image forming apparatus is A3, by setting the circumferential length of an intermediary image bearing member of the image forming apparatus to a size greater than the long side of a recording medium of A3 size and the width of the intermediary image bearing member to a size greater than the short side of the recording medium of A3 size, a color image can be formed on each side of any recording medium used in the image forming apparatus. However, providing such an image forming apparatus including an intermediary image bearing member having such a large surface size results in a higher cost apparatus relative to an image forming apparatus including an intermediary image bearing member having a smaller surface size. 
   Demands of users for an image forming apparatus vary, as described above. Thus, some users only use a recording medium of A4 size as a maximum. If this user can only obtain an expensive image forming apparatus including an intermediary image bearing member having a large surface size corresponding to a recording medium of, e.g., A3 size, the user is forced to bear an unnecessary cost burden in light of their needs. Further, when an intermediary image bearing member having a relatively long circumferential length is used, as compared to using an intermediary image bearing member of shorter circumferential length, the time required for forming an image is increased, resulting in decreased image forming efficiency. 
   Conversely, a user who has obtained an image forming apparatus including an intermediary image bearing member that is small in surface size because of an existing need to form an image on each side of only a small-sized recording medium has a problem if a need to form images on each side of a larger sized recording medium a rises. In this case, the user must obtain a second image forming apparatus that includes an intermediary image bearing member that is of a sufficiently large surface size to meet this new need, and thereby bear an excessive cost burden. The same kind of problem arises when the user who has obtained an image forming apparatus including an intermediary image bearing member having a large circumferential surface no longer needs to form an image on each side of a large-sized recording medium any more and desires to obtain an image forming apparatus including an image bearing member having a small circumferential surface. 
   Another image forming apparatus is known, in which at least one image bearing member is provided, visible images of different colors are formed on the image bearing member, and the visible images of different colors are transferred onto an intermediary transfer member by superimposing one image of different color upon another so that a full color image is formed on the intermediary transfer member. The full color image is then transferred onto a recording medium by a transfer device. The above-described image forming apparatus forms a color image on only one side of a recording medium. Still another image forming apparatus is known, in which a first image formed on an intermediary transfer member by superimposed visible images transferred from an image bearing member is transferred onto an intermediary image bearing member. The first image is then transferred onto a first side of a recording medium and at the same time a second image formed on the intermediary transfer member by subsequent superimposed visible images transferred from the image bearing member is transferred onto the second side of the recording medium. The image forming apparatus having this configuration forms a color image on each side of a recording medium in a relatively short time. 
   The former image forming apparatus forms a color image only on one side of a recording medium, whereas the latter image forming apparatus forms a color image on each side of a recording medium. Therefore, in terms of functions, the latter image forming apparatus is superior to the former image forming apparatus. However, the cost of making the latter image forming apparatus is higher than that of the former image forming apparatus, so that the price thereof is also higher than that of the former image forming apparatus. 
   Also, an image forming apparatus is known, in which visible images are formed on at least one image bearing member and the visible images are immediately transferred by a transfer device onto a recording medium, being superimposed one upon another, thereby forming a color image on the recording medium. Further, another image forming apparatus is known, in which visible images formed on an image bearing member are transferred onto an intermediary image bearing member, being superimposed one upon another, thereby forming a first image thereupon, the first image is then transferred onto one side of a recording medium, and visible images subsequently formed on the image bearing member are transferred onto the other side of the recording medium, being superimposed one upon another, thereby forming a second image on the other side of the recording medium. In this case also, the former image forming apparatus forms a color image only on one side of a recording medium, whereas the latter image forming apparatus forms a color image on each side of a recording medium. Therefore, in terms of functions, the latter image forming apparatus is superior to the former image forming apparatus. However, the cost of making the latter image forming apparatus is higher than that of the former image forming apparatus, so that the price thereof is higher than that of the former image forming apparatus. 
   As described above, demands of users for an image forming apparatus are diverse because while one user desires to obtain an image forming apparatus capable of forming an image on each side of a recording medium at a high speed even if the price is high, while another user prefers an inexpensive image forming apparatus even if an image cannot be formed on each side of a recording medium. The former user obtains an image forming apparatus capable of forming an image on each side of a recording medium, and the latter user obtains an image forming apparatus capable of forming an image only on one side of a recording medium. However, the latter user who has obtained an image forming apparatus capable of forming an image only on one side of a recording medium may be later required to obtain an image forming apparatus capable of forming an image on each side of a recording medium because of a change in needs. In this case, this user must purchase a second image forming apparatus that is capable of forming an image on each side of a recording medium, being forced to bear a great financial burden as to purchasing two different image forming apparatuses. 
   SUMMARY OF THE INVENTION 
   The present invention has been made in view of the above-discussed and other problems and addresses the above-discussed and other problems. 
   Preferred embodiments of the present invention provide an image forming apparatus forming an image one each side of a recording medium, that can meet different demands of a user at a relatively low cost, a belt unit for use in the image forming apparatus, and an image forming system using the image forming apparatus. 
   According to a preferred embodiment of the present invention, an image forming apparatus for forming an image on a recording medium includes a main body housing, a recording medium conveying path formed in the main body housing, and an installing part provided in the main body housing along a part of the recording medium conveying path and configured such that either of at least two devices having different functions that are performed when each is detachably and alternatively easily installed in the installing part of the main body housing. The image forming apparatus can further include a receiving and guiding mechanism provided to the main body housing and configured to receive and guide either of the at least two devices having different functions to be installed in and removed from the installing part of the main body housing. 
   According to another preferred embodiment of the present invention, an image forming apparatus includes at least one image bearing member on which a visible image is formed, an intermediary transfer member onto which visible images of different colors formed on the at least one image bearing member are transferred to be superimposed one upon another, thereby forming a first image thereupon, and an installing part provided in a main body housing of the apparatus and configured such that either of at least two intermediary image bearing members different in size can be easily installed and removed, wherein each of the intermediary image bearing members of different size is configured such that when installed in the installing part of the main body housing of the apparatus, will have the first image transferred thereupon. The image forming apparatus is configured such that when the either of at least two intermediary image bearing members different in size is installed in the installing part of the main body housing of the apparatus, the first image on the installed either of at least two intermediary image bearing members is transferred onto one side of a recording medium and a second image formed on the intermediary transfer member by another visible images subsequently transferred from the at least one image bearing member being superimposed one upon another is transferred onto the other side of the recording medium. The image forming apparatus can further include a receiving and guiding mechanism provided to the main body housing of the apparatus that is configured to receive and guide any of at least two intermediary image bearing members of different size being installed in or removed from the installing part of the main body housing of the apparatus. 
   According to still another preferred embodiment of the present invention, an image forming apparatus includes at least one image bearing member on which a visible image is formed, an intermediary transfer member onto which visible images of different colors formed on the at least one image bearing member are transferred to be superimposed one upon another, thereby forming a first image thereupon, and an installing part provided in a main body housing of the apparatus and configured such that either of a transfer device configured such that when installed in the installing part of the main body housing of the apparatus, the first image formed on the intermediary transfer member is transferred onto a recording medium, or an intermediary image bearing member configured such that when installed in the installing part of the main body housing of the apparatus, the first image formed on the intermediary transfer member is transferred thereupon, can be easily installed therein or removed therefrom in a detachable manner. The image forming apparatus is configured such that when the intermediary image bearing member is installed in the installing part of the main body housing of the apparatus, the first image transferred onto the intermediary image bearing member from the intermediary transfer member is transferred onto a first side of the recording medium and a second image formed on the intermediary transfer member by other visible images of different colors, subsequently transferred from the at least one image bearing member being superimposed one upon another, is transferred onto a second side of the recording medium. The image forming apparatus may further include a receiving and guiding mechanism provided to the main body housing of the apparatus that is configured to receive and to guide the either of a transfer device or an intermediary image bearing member being installed in and being removed from the installing part of the main body housing. 
   According to still another preferred embodiment of the present invention, an image forming apparatus includes at least one image bearing member on which a visible image is formed, and an installing part provided in a main body housing of the apparatus and configured such that either of a transfer device configured such that when installed in the installing part of the main body housing of the apparatus, the visible image formed on the at least one image bearing member is transferred onto a recording medium, or an intermediary image bearing member configured such that when installed in the installing part of the main body housing of the apparatus, visible images formed on the at least one image bearing member are transferred thereon to be superimposed one upon another, thereby forming a first image thereupon, can be easily installed therein or removed therefrom in a detachable manner. The image forming apparatus is configured such that when the intermediary image bearing member is installed in the installing part of the main body housing of the apparatus, the first image on the intermediary image bearing member is transferred onto a first side of the recording medium and a second image formed by another visible images subsequently formed on the at least one image bearing member is transferred onto a second side of the recording medium. The image forming apparatus may further include a receiving and guiding mechanism provided to the main body housing of the apparatus and configured to receive and to guide the either of a transfer device and an intermediary image bearing member to be installed in or removed from the installing part of the main body housing. 
   According to still another preferred embodiment of the present invention, a belt unit includes an intermediary image bearing member formed as an endless belt, and a plurality of supporting rollers supporting the intermediary image bearing member formed in an endless belt. The intermediary image bearing member formed as an endless belt and the plurality of supporting rollers supporting the intermediary image bearing member are integrated to form a belt unit, and the belt unit is configured to be installed in the installing part of the main body housing of any of the above-described image forming apparatuses. 
   According to still another preferred embodiment of the present invention, an image forming system includes any of the above-described image forming apparatuses and a host computer connected with the image forming apparatus. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete appreciation of the present invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in conjunction with accompanying drawings, wherein: 
       FIG. 1  is a vertical sectional view of an image forming apparatus configured to function as a printer according to an embodiment of the present invention; 
       FIG. 2  is an enlarged diagram of a cleaning device for an intermediary transfer member of the image forming apparatus; 
       FIG. 3  is an enlarged diagram of neighborhood of a contacting part of the intermediary transfer member and an intermediary image bearing member of the image forming apparatus; 
       FIG. 4  is an exploded perspective view of a belt unit including the intermediary image bearing member of the image forming apparatus; 
       FIG. 5A  is a perspective view illustrating an outer appearance of another belt unit including another intermediary image bearing member that is longer in circumferential length; 
       FIG. 5B  is a perspective view illustrating an outer appearance of the belt unit installed in the image forming apparatus of  FIG. 1 ; 
       FIG. 5C  is a perspective view illustrating an outer appearance of still another belt unit including another intermediary image bearing member that is smaller in width; 
       FIG. 5D  is a perspective view of the image forming apparatus, in which a movable housing of the main body housing is opened and a belt unit has been removed from an installing part of the main body housing, illustrating an example of a receiving and guiding mechanism configured to receive and guide either of a plurality of belt units including intermediary image bearing members different in size to be installed in and removed from an installing part of the main body housing; 
       FIG. 6  is a vertical cross section of the image forming apparatus in which the another belt unit including another intermediary image bearing member that is longer in circumferential length is installed; 
       FIG. 7  is a vertical cross section of a part of the image forming apparatus, illustrating a state that the movable housing including the intermediary image bearing member is rotated to a first opened position; 
       FIG. 8  is a cross section illustrating the belt unit removed from the main body housing of the image forming apparatus; 
       FIG. 9  is a cross section illustrating the another belt unit including another intermediary image bearing member that is longer in circumferential length, removed from the main body housing of the image forming apparatus; 
       FIG. 10  is a perspective view of an image forming system in which image forming apparatuses are connected with a host computer; 
       FIG. 11  is a vertical cross section illustrating a state that the movable housing including the belt unit is moved in a horizontal direction to be removed from a fixed housing of the main body housing; 
       FIG. 12  is a vertical cross section illustrating a state that the movable housing illustrated in  FIG. 7  is further rotated to a second opened position; 
       FIG. 13  is a perspective view of the image forming apparatus including a movable housing that bends; 
       FIG. 14  is a diagram for explaining a mechanism of the movable housing of  FIG. 13 ; 
       FIG. 15  is a diagram illustrating an exemplary configuration of the image forming apparatus in which a transfer device for transferring the first image formed on the intermediary image bearing member onto one side of a recording medium is arranged inside of the intermediary image bearing member; 
       FIG. 16  is a diagram illustrating another exemplary configuration of the image forming apparatus in which a transfer device for transferring the first image on the intermediary image bearing member onto one side of a recording medium is arranged inside of the intermediary image bearing member; 
       FIG. 17A  is a diagram of the another intermediary image bearing member that is longer in circumferential length in a developed state, illustrating a size thereof and a size of a large recording medium; 
       FIG. 17B  is a diagram of the intermediary image bearing member that is shorter in circumferential length in a developed state, illustrating a size thereof and a size of a small recording medium; 
       FIG. 18  is a diagram illustrating a case that images for two pages are formed on the intermediary image bearing member longer in circumferential length in a developed state; 
       FIG. 19  is a vertical sectional view of the image forming apparatus including a single image bearing member, according to another embodiment of the present invention; 
       FIG. 20  is a vertical sectional view of the image forming apparatus according to still another embodiment of the present invention; 
       FIG. 21  is a vertical sectional view of the image forming apparatus illustrated in  FIG. 20 , in which the movable housing is opened; 
       FIG. 22  is a vertical sectional view of the image forming apparatus according to still another embodiment of the present invention; 
       FIG. 23  is a vertical sectional view of the image forming apparatus illustrated in  FIG. 22 , in which the movable housing is opened; 
       FIG. 24  is a perspective view of an image forming system including the image forming apparatuses illustrated in  FIG. 20  and  FIG. 22 ; 
       FIG. 25  is a vertical sectional view of the image forming apparatus according to still another embodiment of the present invention; 
       FIG. 26  is a vertical sectional view of the image forming apparatus different in configuration from the image forming apparatus illustrated in  FIG. 25 ; 
       FIG. 27  is a vertical sectional view of the image forming apparatus according to still another embodiment of the present invention; 
       FIG. 28  is a vertical sectional view of the image forming apparatus different in configuration from the image forming apparatus illustrated in  FIG. 27 ; 
       FIG. 29  is a diagram of another belt unit; 
       FIG. 30  is a partial sectional view of the image forming apparatus of  FIG. 1 , in which still another belt unit including the intermediary image bearing member that is short in circumferential length is installed in the main body housing according to another example of the receiving and guiding mechanism; 
       FIG. 31  is a partial sectional view of the image forming apparatus, in which still another belt unit including the intermediary image bearing member long in circumferential length is installed in the main body housing according to the another example of the receiving and guiding mechanism; 
       FIG. 32  is a sectional view for explaining each of the still another belt units in  FIG. 30  and  FIG. 31  is installed in and removed from the installing part of the main body housing; and 
       FIG. 33  is a perspective view for explaining the another example of the receiving and guiding mechanism. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout several views, preferred embodiments of the present invention are described. 
     FIG. 1  is a vertical sectional view of an image forming apparatus configured to function as a printer according to an embodiment of the present invention. The image forming apparatus includes a plurality of image bearing members arranged inside of a main body housing  1  of the image forming apparatus. In this example, four image bearing members are provided, a first image bearing member  2 Y, a second image bearing member  2 M, a third image bearing member  2 C, and a fourth image bearing member  2 BK. Visible toner images of different colors are formed on the image bearing members  2 Y,  2 M,  2 C, and  2 BK, respectively. Which color&#39;s toner image is to be formed on which image bearing member may be appropriately determined. In this example, yellow toner images, magenta toner images, cyan toner images, and black toner images are formed on the first through fourth image bearing members  2 Y,  2 M,  2 C and  2 BK, respectively. 
   A drum-like shaped photo conductor is used for each of the image bearing members  2 Y,  2 M,  2 C and  2 BK in this example. A photo conductor formed as an endless belt and spanned around a plurality of rollers to be rotated may be also used for each of the image bearing members  2 Y,  2 M,  2 C and  2 BK. A drum-like shaped image bearing member may be configured such that a layer of a photoconductive organic semiconductor is provided on a surface of a cylindrical base body of aluminum about 30 to 100 mm in diameter. It is needless to say that the drum-like shaped image bearing member may be configured otherwise. 
   Further, as described later, the image forming apparatus may be configured to include only one image bearing member. Thus, the present invention can be applied to any image forming apparatus having at least one image bearing member. 
   An intermediary transfer member  3  is arranged to oppose the first through fourth image bearing members  2 Y,  2 M,  2 C and  2 BK, and each of the image bearing members  2 Y,  2 M,  2 C and  2 BK contacts a surface of the intermediary transfer member  3 . As the intermediary transfer member  3 , a drum-like shaped member may be also used. However, in the example illustrated in  FIG. 1 , an endless belt spanned around supporting rollers  4  and  5  and configured to be driven to rotate in a direction of an arrow A is used for the intermediary transfer member  3 . Visible toner images of respective colors formed on the image bearing members  2 Y,  2 M,  2 C, and  2 BK are transferred onto the intermediary transfer member  3  to be superimposed one upon another. 
   First through fourth image forming devices  6 Y,  6 M,  6 C and  6 BK are provided to the image bearing members  2 Y,  2 M,  2 C, and  2 BK to form visible images thereupon, respectively. Configurations and operations of the image forming devices  6 Y,  6 M,  6 C and  6 BK for forming visible images on the image bearing members  2 y,  2 M,  2 C and  2 BK are substantially the same except that colors of the visible images are different. Also, configurations and operations of transfer rollers  12  provided to the image bearing members  2 Y,  2 M,  2 C, and  2 BK for transferring the visible images onto the intermediary transfer member  3 , respectively, are substantially the same. Therefore, description of the configurations and operations of the image forming devices  6 Y,  6 M,  6 C and  6 BK and the transfer rollers  12  will only be made with respect to an operation of forming a toner image on the first image bearing member  2 Y and transferring the toner image onto the intermediary transfer member  3 . 
   The image bearing member  2 Y is rotated in a counterclockwise direction in  FIG. 1 . At this time, a light from a discharging device  7  is irradiated on a surface of the image bearing member  2 Y so that a surface potential of the image bearing member  2 Y is initialized. The initialized surface of the image bearing member  2 Y is uniformly charged to a predetermined polarity, in this example to a minus polarity, by a charging device  8 . A light emitted by an exposure device  9  is irradiated on the charged surface of the image bearing member  2 Y, so that an electrostatic latent image corresponding to writing information is formed on the image bearing member  2 Y. In the example illustrated in  FIG. 1 , the exposure device  9  including an LED array and an imaging device is used. However, a laser writing device configured to emit a modulated laser light may be also used for the exposure device  9 . 
   The electrostatic latent image formed on the image bearing member  2 Y is developed to a visible yellow toner image when passing a developing device  11 . The developing device  11  in this example includes a developing roller  11 A bearing and conveying a dry-type developer, and the developing device  11  is configured to develop an electrostatic latent image on the image bearing member  2 Y into a visible image with the dry-type developer born by the developing roller  11 A. A developing device using a wet-type developer can also be used. 
   The transfer roller  12  as an example of a transfer device and a backing roller  13  are arranged inside of the intermediary transfer member  3  formed as an endless belt at positions where the transfer roller  12  and the backing roller  13  substantially oppose the image bearing member  2 Y, sandwiching the intermediary transfer member  3  with the image bearing member  2 Y and the transfer roller  12  and the backing roller  13 . The transfer roller  12  and the backing roller  13  contact a backside of the intermediary transfer member  3 , so that a suitable transfer nip is securely formed by the image bearing member  2 Y and the intermediary transfer member  3 . Another transfer device, such as a transfer brush, a transfer blade, or a corona discharger, may be also used for performing the function of the transfer roller  12 . However, when the transfer roller  12  is used, transfer efficiency may be enhanced by pressing the intermediary transfer member  3  to the image bearing member  2 Y with the transfer roller  12  when transferring a visible image on the image bearing member  2 Y onto the intermediary transfer member  3 . 
   A transfer voltage having a polarity opposite to the charging polarity of a visible toner image formed on the image bearing member  2 Y is applied to the transfer roller  12 . In this example, a transfer voltage of a plus polarity is applied. Thereby, a transfer electric field is formed between the image bearing member  2 Y and the intermediary transfer member  3 , so that a toner image formed on the image bearing member  2 Y is electrostatically transferred onto the intermediary transfer member  3  rotated in synchronism with the image bearing member  2 Y. Residual toner remaining on a surface of the image bearing member  2 Y after transferring the toner image onto the intermediary transfer member  3  is removed by a cleaning member of a cleaning device  14 , so that the surface of the image bearing member  2 Y is cleaned. 
   As described above, visible images maybe repeatedly formed on the image bearing member  2 Y by charging, exposing, developing and cleaning operations of the image forming device  6 Y. It is needless to say that an appropriate image forming device other than the one described above and illustrated in  FIG. 1  may be also used. 
   In substantially similar manners, visible magenta toner images, cyan toner images, and black toner images are formed respectively on the second through fourth image bearing members  2 M,  2 C and  2 BK by the second through fourth image forming devices  6 M,  6 C and  6 BK. The developing devices  6 M,  6 C and  6 BK arranged to oppose the image bearing members  2 M,  2 C and  2 BK form toner images of different colors on the image bearing members  2 M,  2 C and  2 BK, respectively. The toner images of different colors formed on the image bearing members  2 M,  2 C and  2 BK are electrostatically transferred by the transfer rollers  12 , arranged to oppose the image bearing members  2 M,  2 C and  2 BK sandwiching the intermediary transfer member  3  with the image bearing members  2 M,  2 C and  2 BK and the transfer rollers  12 , respectively, onto the intermediary transfer member  3  on which a yellow toner image has been previously transferred to be sequentially superimposed one upon another. Thus, an image formed by superimposed visible toner images of different colors is formed on the intermediary transfer member  3 . The image is herein referred to as a first image. 
   A seamless heat-resisting belt having resistance enabling transferring of toner images from the image bearing members  2 Y,  2 M,  2 C and  2 BK and bearing of the toner images thereupon is used for the intermediary transfer member  3 . For example, a belt configured such that a surface layer of a low surface energy is formed on a belt base of a resin film or rubber 50–500 μm in thickness may be used for the intermediary transfer member  3 . The volume resistivity of the entire part of the belt may be set, for example, to 10 6 –10 12  Ω cm, and the surface resistivity thereof, for example, to 10 5 –10 12  Ω/□. 
   An intermediary image bearing member  15  onto which the first image formed on the intermediary transfer member  3  by transferred superimposed visible images is arranged at the left side of the intermediary transfer member  3  in  FIG. 1 . The intermediary image bearing member  15  may be formed in a drum-like shape and be configured to be driven to rotate. In this example, the intermediary image bearing member  15  is configured by an endless belt spanned around a plurality of supporting rollers  16 ,  17 ,  18  and  19  and rotated in a direction of an arrow B in synchronism with the intermediary transfer member  3 . 
   A transfer roller  20  as an example of a transfer device is arranged inside of the intermediary image bearing member  15  near the supporting roller  19 . In the example illustrated in  FIG. 1 , the transfer device for the intermediary image bearing member  15  is also constituted by a roller, i.e., the transfer roller  20  contacting a backside of the intermediary image bearing member  15 , and the supporting roller  5  for the intermediary transfer member  3  is pressed to contact apart of the intermediary image bearing member  15  between the transfer roller  20  and the supporting roller  19 . Thereby, the intermediary transfer member  3  and the intermediary image bearing member  15  contact each other, forming an appropriate nip between them. Other members such as a corona discharging device, a transfer brush, a transfer blade, etc., maybe also used for the transfer device. 
   A transfer voltage of a polarity opposite to the toner charging polarity of the first image on the intermediary transfer member  3  (in this example, a positive polarity) is applied to the transfer roller  20  provided inside of the intermediary image bearing member  15 , so that a transfer electric field is formed between the intermediary transfer member  3  and the intermediary image bearing member  15 . Thereby, when the first image on the intermediary transfer member  3  reaches a transfer position between the intermediary transfer member  3  and the intermediary image bearing member  15 , the first image is electrostatically transferred onto the intermediary image bearing member  15 . Thus, the transfer roller  20  performs a function of transferring the first image, which has been formed on the intermediary transfer member  3  by visible images transferred from the plurality of image bearing members  2 Y,  2 M,  2 C and  2 BK to be superimposed one upon another, onto the intermediary image bearing member  15 . Residual toner adhering to a surface of the intermediary transfer member  3  after transfer of the first image onto the intermediary image bearing member  15  is removed, together with paper dust, etc., by a cleaning device  22 . 
   When a surface of the intermediary transfer member  3  moves to reach a predetermined position, in substantially the same manner as described above, subsequent visible toner images of yellow, magenta, cyan and black colors are sequentially started to be formed on the plurality of image bearing members  2 Y,  2 M,  2 C and  2 BK, and these visible images are electrostatically transferred onto the intermediary transfer member  3  in sequence so as to be superimposed one upon another. An image thus formed by visible images superimposed each other on the intermediary transfer member  3  is herein referred to as a second image. 
   A sheet feeding device  23  is provided at a lower part of the main body housing  1 . The sheet feeding device  23  includes a sheet feeding cassette  24  stacking and accommodating therein a recording medium P, e.g., a transfer sheet, a resin sheet, etc., a feeding roller  25  to feed out the recording medium P from the sheet feeding cassette  24 , and a separating pad  57  preventing double-feeding of the recording medium P. The feeding roller  25  rotates while contacting an upper surface of a top recording medium P in the sheet feeding cassette  24 , thereby the top recording medium P being fed out to a registration roller pair  26 . 
   The registration roller pair  26  feeds out the recording medium P upward at a timing that the second image on the intermediary transfer member  3  and the first image on the intermediary image bearing member  15  are transferred onto respective surfaces of the recording medium P to be aligned with each other. The recording medium P thus fed out by the registration roller pair  26  passes the nip part of the intermediary transfer member  3  and the intermediary image bearing member  15 , being rotated in the arrow directions A and B respectively in synchronism with each other, to be conveyed upward, and while being conveyed, the first image on the intermediary image bearing member  15  is electrostatically transferred onto one side of the recording medium P and the second image on the intermediary transfer member  3  is electrostatically transferred onto the other side of the recording medium P. The timing of feeding out the recording medium P from the registration roller pair  26  may be controlled, for example, by reading a mark (not shown), which is provided on each of the intermediary transfer member  3  and the intermediary image bearing member  15  as a criterion for image formation, with a sensor (not shown). 
   The order of starting transfer of the first image and the second image onto respective sides of a recording medium P may be arbitrary, or transfer of the first image onto one side and the second image onto the other side of a recording medium P may be started at the same time. In this example, first, the second image on the intermediary transfer member  3  starts to be transferred onto the other side of the recording medium P, and then the first image on the intermediary image bearing member  15  starts to be transferred onto the one side of the recording medium P. That is, when the recording medium P passes between the intermediary transfer member  3  and the intermediary image bearing member  15 , a transfer voltage of a polarity (in the example, a plus polarity) that is opposite to the toner charging polarity of the second image on the intermediary transfer member  3  is applied to the transfer roller  20 , and thereby the second image on the intermediary transfer member  3  is electrostatically transferred onto the other side of the recording medium P. The recording medium P onto which the second image has been transferred is continuously conveyed upward while being in close contact with a surface of the intermediary transfer member  3 . At this time, a transfer voltage of a polarity (in this example, a plus polarity) that is opposite to the toner charging polarity of the first image on the intermediary image bearing member  15  is applied to a corona discharging device  27  as an example of a transfer device arranged to be separated from a surface of the intermediary image bearing member  15 . Thereby, a transfer electric field is formed between the intermediary image bearing member  15  and the recording medium P, and the first image on the intermediary image bearing member  15  is electrostatically transferred onto the other side of the recording medium P. 
   The corona discharging device  27  is a known discharging device having a corona wire to which a transfer voltage is applied. The corona discharging device  27  is arranged in a position separated from a surface of the intermediary image bearing member  15  not to contact a recording medium P being conveyed in close contact with the intermediary image bearing member  15 . Accordingly, the second image on the other side of the recording medium P is never disturbed by the corona discharging device  27 . Another transfer device other than the corona discharging device  27  may be used as long as it has substantially the same function as that of the corona discharging device  27 . An opposing electrode  63  is arranged for the corona discharging device  27  sandwiching the intermediary image bearing member  15  with the corona discharging device  27  and the opposing electrode  63 . The opposing electrode  27  is grounded. In this example, a roller that rotates while contacting an internal side of the intermediary image bearing member  15  is used for the opposing electrode  63 , however, a plate may be used for the opposing electrode  63 . 
   Separate transfer devices may be arranged for transferring the first image on the intermediary transfer member  3  onto the intermediary image bearing member  15  and for transferring the second image on the intermediary transfer member  3  onto the other side of the recording medium P, respectively. However, in this example, the transfer roller  20  performs both the transferring of the first image and the transferring of the second image so that the configuration of the image forming apparatus is simplified. 
   The recording medium P onto which the first image and the second image have been transferred is conveyed further upward after having been separated from the intermediary image bearing member  15 , and passes between a pair of fixing rollers  29  and  30  of a fixing device  28 , rotating in directions indicated by arrows, respectively. The fixing rollers  29  and  30  are heated by a heater (not shown). Heat and pressure are applied to both of the first image and the second image transferred onto the recording medium P, and thereby both images are fixed to the recording medium P at one time. The recording medium P passed the fixing device  28  is expelled, while being guided by a sheet expelling guide  52 , onto a sheet expelling part  31  by a sheet expelling roller pair  53  as indicated by an arrow C. 
   A surface of the intermediary transfer member  3  after transfer of the second image onto the recording medium P is cleaned by the cleaning device  22 . Similarly, residual toner adhering to a surface of the intermediary image bearing member  15  after transfer of the first image to the recording medium P is removed by a cleaning device  32 , so that the surface of the intermediary image bearing member  15  is cleaned. 
   The cleaning device  32  includes a case  67  thereof, a cleaning blade  54  as an example of a cleaning member arranged inside of and supported by the case  67 , and a toner conveying member  55 . The entire part of the cleaning device  32  is supported by a rotating fulcrum  56  to rotate around the fulcrum  56 , so that the cleaning blade  54  contacts and separates from a surface of the intermediary image bearing member  15 . The cleaning blade  54  of the cleaning device  32  separates from a surface of the intermediary image bearing member  15  when the first image born on the intermediary image bearing member  15  passes the cleaning blade  54  and contacts a surface of the intermediary image bearing member  15  when removing residual toner adhering to the surface of the intermediary image bearing member  15 . 
   Residual toner adhering to a surface of the intermediary image bearing member  15  is scraped to be removed when the cleaning blade  54  contacts a surface of the intermediary image bearing member  15 , and the toner removed from the intermediary image bearing member  15  is collected into a residual toner collecting device (not shown) by the toner conveying member  55 . The supporting roller  18  supporting the intermediary image bearing member  15  is arranged to oppose the cleaning blade  54  sandwiching the intermediary image bearing member  15  with the supporting roller  18  and the cleaning blade  54 . When the cleaning blade  54  is pressed to contact a surface of the intermediary image bearing member  15 , the cleaning blade  54  is pressed to the supporting roller  18  via the intermediary image bearing member  15 . Thus, the supporting roller  18  functions as a backing member for the cleaning blade  54 . 
   An appropriate device can also be used for the cleaning device  22  for the intermediary transfer member  3 . The cleaning device  22  illustrated in  FIG. 1  includes, as illustrated in an enlarged diagram of  FIG. 2 , a case  58 , a brush roller  59  as an example of a cleaning member rotatably supported by the case  58 , and a bias roller  60  also rotatably supported by the case  58  and contacting the brush roller  59 . The brush roller  59  is rotated in the direction indicated by an arrow while contacting the intermediary transfer member  3 , and the bias roller  60  is also driven to rotate in the direction indicated by an arrow. A voltage having a polarity opposite to the charging polarity of residual toner on the intermediary transfer member  3  is applied to the bias roller  60 . Thereby, the residual toner on the intermediary transfer member  3 , while being scraped off the intermediary transfer member  3 , electrostatically moves to the brush roller  59 , and then the toner electrostatically moves to the bias roller  60 . Further, the toner on the bias roller  60  is scraped off the bias roller  60  by a blade  61  pressed to a surface of the bias roller  60  to be conveyed by a toner conveying member  62  to a toner collecting part not shown. The brush roller  59  always contacts a surface of the intermediary transfer member  3 . 
   An endless belt having resistance enabling transfer of a toner image from the intermediary transfer member  3  and transfer of the toner image from the intermediary transfer member  3  to a recording medium P is used for the intermediary image bearing member  15 . For example, a belt having a releasing-type coating layer of a low surface energy including fluorine resin (e.g., PFA), etc., formed on a belt base of polyimide or polyamide-imide about 50–500 μm in thickness can be used for the intermediary image bearing member  15 . The volume resistivity of the entire intermediary image bearing member  15  can be preferably set to 10 6 –10 12  Ω·cm, and the surface resistivity thereof can be preferably set to 10 5 –10 12  Ω/□. By configuring the intermediary image bearing member  15  to include such a releasing-type coating layer on its surface, the transferring property of the intermediary image bearing member  15  in transferring the first image on the intermediary image bearing member  15  onto a recording medium P and the cleaning property of the surface of the intermediary image bearing member  15  after transfer of the first image can be enhanced. Further, by setting the surface resistivity of the intermediary image bearing member  15  to 10 5 –10 12  Ω/□, the first image can be securely born on a surface of the intermediary image bearing member  15 . 
     FIG. 3  is an enlarged diagram of neighborhood of a contacting part of the intermediary transfer member  3  and the intermediary image bearing member  15 . Because a transfer voltage must be applied to the transfer roller  20 , a metal roller is used for the transfer roller  20 , the diameter of which is about 10 mm. The supporting roller  19  of the intermediary image bearing member  15  is also a metal roller, and is grounded to prevent frictional charging between the intermediary image bearing member  15  and a recording medium P so that transfer of an image can be satisfactorily performed. The diameter of the supporting roller  19  is about 16 mm. Further, for preventing electric current from leaking to the supporting roller  19 , the distance D between the supporting roller  19  and the transfer roller  20  should be preferably equal to or greater than 5 mm. In the example illustrated in  FIG. 3 , the distance D is set to about 8 mm. 
   The intermediary image bearing member  15  may be configured to be supported by the supporting roller  16  to swing around the supporting roller  16 , so that the intermediary image bearing member  15  can contact and separate from the intermediary transfer member  3 . Thereby, when transferring the first image on the intermediary transfer member  3  onto the intermediary image bearing member  15  and when transferring the second image on the intermediary transfer member  15  onto a recording medium P, the intermediary transfer member  3  and the intermediary image bearing member  15  are brought into contact directly or via the recording medium P, and the intermediary transfer member  3  and the intermediary image bearing member  15  are separated from each other at other times. 
   In order to obtain both of the first image transferred onto one side of a recording medium P and the second image transferred onto the other side of the recording medium P in a correct direction (not upside down), when forming toner images for the first image on the image bearing members  2 Y,  2 M,  2 C and  2 BK, respective latent images must be formed upside down on the image bearing members  2 Y,  2 M,  2 C and  2 BK, and when forming toner images for the second image on the image bearing members  2 Y,  2 M,  2 C and  2 BK, respective latent images must be formed in a correct direction (not upside down) on the image bearing members  2 Y,  2 M,  2 C and  2 BK. Switching of exposure for enabling the above-described formation of latent images is controlled by a writing controller (not shown). 
   Further, when the sheet expelling part  31  is arranged as illustrated in  FIG. 1 , a recording medium P is stacked on the sheet expelling part  31  with the other side of the recording medium P on to which the second image has been directly transferred from the intermediary transfer member  3  faced down. Therefore, in order to put the page of the recording medium P fed onto the sheet expelling part  31  in order, toner images must be formed on the image bearing members  2 Y,  2 M,  2 C and  2 BK such that the first image is on the second page and the second image is on the first page. That is, the order of forming the first and second images is controlled such that the first image is on an even page and the second image on an odd page, and at the same time the sheet conveying path for conveying a recording medium P is configured such that the recording medium P is fed onto the sheet expelling part  31  with the second image face down. With the above-described configuration, even when a large number of sheets of the recording medium P are fed onto the sheet expelling part  31  consecutively, the pages thereof can be put in order using a known technology of storing image data in a memory. 
   Each of the image bearing members  2 Y,  2 M,  2 C and  2 BK, and the charging device  8 , the developing device  11 , and the cleaning device  14  arranged around each of the image bearing members  2 Y,  2 M,  2 C and  2 BK may be integrated to be configured as a process cartridge, so that the process cartridge can be replaced with a new one when the life of the process cartridge ends. 
   Toner containers  36 Y,  36 M,  36 C and  36 BK containing yellow toner, magenta toner, cyan toner, and black toner, respectively, are provided in a toner container accommodating space S above the intermediary transfer member  3  and below the sheet expelling part  31 . Toner contained in each of the toner containers  36 Y,  36 M,  36 C and  36 BK is replenished to a corresponding developing device  11  by a powder pump (not shown). 
   A power source unit E 1  and a control unit E 2  are also provided inside of the main body housing  1 . Further, a fan F 1  is provided to discharge air to prevent the temperature inside of the main body housing  1  from being excessively raised. 
   An operation panel OP is provided to the main body housing  1 . The operation panel OP is provided with display lamps for displaying conditions and states of the image forming apparatus and other devices connected with the image forming apparatus, and key buttons used for inputting various instructions. 
   It is possible to form a color image only on one side of a recording medium P with the image forming apparatus of  FIG. 1 . In this case, toner images of different colors formed on the plurality of image bearing members  2 Y,  2 M,  2 C and  2 BK are transferred onto the intermediary transfer member  3  to be superimposed one upon another, so that a superimposed visible image is formed on the intermediary transfer member  3 . The superimposed visible image is then transferred onto one side of a recording medium P conveyed from the sheet feeding device  23  by a function of the transfer roller  20 . The recording medium P is conveyed upward by being in close contact with a surface of the intermediary image bearing member  15  which is moving. At this time, the corona discharging device  27  acting as a transfer device is not operated. When the recording medium P passes the fixing device  28 , the transferred image is fixed to the recording medium P, and subsequently the recording medium P is fed onto the sheet expelling part  31 . At this time, the recording medium P is fed onto the sheet expelling part  31  with the side thereof on which the image is fixed face down. Therefore, when the above-described image forming operations are repeated, the pages of a plurality of recording medium P fed onto the sheet expelling part  31  are in order. Thus, a mode for obtaining a recorded image on one side of a recording medium, in which a visible image transferred from an image bearing member onto an intermediary transfer member is transferred, without being transferred onto an intermediary image bearing member, onto the one side of the recording medium, can be realized. 
   Further, a color image may be formed only on one side of a recording medium P by transferring a visible image formed on the intermediary transfer member  3  by toner images transferred onto the intermediary transfer member  3  from the plurality of image bearing members  2 Y,  2 M,  2 C and  2 BK, onto the intermediary image bearing member  15 . The visible image on the intermediary image bearing member  15  is transferred onto the recording medium P, and after the visible image on the recording medium P is fixed by the fixing device  28 , the recording medium P is fed onto the sheet expelling part  31 . A fixing temperature of the fixing device  28  when forming an image on only one side of a recording medium P can be set lower than when forming an image on each side of the recording medium P. 
   Furthermore, the image forming apparatus of  FIG. 1  can form an image of a single color or an image of a plurality of colors by forming visible images only on some of the plurality of image bearing members  2 Y,  2 M,  2 C and  2 BK and transferring the visible images onto one side or both sides of a recording medium P in substantially the same manner as described above. 
   In the image forming apparatus of  FIG. 1 , the sheet feeding cassette  24  accommodating the recording medium P as described above is installed in the main body housing  1 , together with the separating pad  57 , such that the sheet feeding cassette  24  can be drawn in the direction of an arrow E relative to the main body housing  1 . In the example illustrated in  FIG. 1 , when the sheet feeding cassette  24  is drawn out, the sheet feeding roller  25  remains inside of the main body housing  1 . By thus drawing out the sheet feeding cassette  24  from the main body housing  1 , a recording medium P can be replenished to the sheet feeding cassette  24 . After replenishing of a recording medium P, by pushing the sheet feeding cassette  24  in the reverse direction relative to the direction of the arrow E, the sheet feeding cassette  24  can be installed in the main body housing  1 . 
   Further, as illustrated in  FIG. 1  by dashed lines, a conveying path  73  for a recording medium P may be formed to pass the sheet feeding cassette  24  in an up-and-down direction. Thereby, when another sheet feeding cassette is provided below the sheet feeding cassette  24 , a recording medium fed from the another sheet feeding cassette can be fed to the registration roller pair  26  by causing the recording medium to pass the conveying path  73 . 
   In the present invention, at least the intermediary image bearing member  15  and the plurality of supporting rollers  16 ,  17 ,  18  and  19  around which the intermediary image bearing member  15  is spanned are integrated to be configured as a belt unit  64 . More specifically, as illustrated in  FIG. 4 , end parts of the supporting rollers  16 ,  17 ,  18  and  19  are rotatably supported by both side plates  65 , and a screw  68  is fixed to one end of an axis of the supporting roller  16 . 
   In the example illustrated in  FIG. 4 , both longitudinal ends of axes of the transfer roller  20  as an example of a transfer device and the opposing electrode  63  constituted by a roller contacting an internal side of the intermediary image bearing member  15  are also rotatably supported by the both side plates  65 , so that the transfer roller  20  and the opposing electrode  63  are also parts of the belt unit  64 . 
   Furthermore, the belt unit  64  includes a cover  66  covering components of the belt unit  64 , such as the intermediary image bearing member  15 , the supporting rollers  16 ,  17 ,  18  and  19 , etc. An exemplary outer appearance of the belt unit  64  is illustrated in  FIG. 5B . The screw  68  and one end of the axis of the transfer roller  20  protrude outside of the cover  66 , and in a state that the belt unit  64  is installed in an installing part (described later) of the main body housing  1 , the screw  68  engages with a driving gear  69  ( FIG. 5D ) provided to the main body housing  1 , so that the supporting roller  16  is driven to rotate via the screw  69  and the screw  68 . Thereby, the intermediary image bearing member  15  is rotated in the direction of the arrow B in  FIG. 1 . Further, in the state that the belt unit  64  is installed in the installing part of the main body housing  1 , the axis of the transfer roller  20  contacts an outputting terminal of a power source (not shown), and a predetermined transfer voltage is applied to the axis of the transfer roller  20 . 
   In the image forming apparatus illustrated in  FIG. 1 , the cleaning device  32  for cleaning a surface of the intermediary image bearing member  15  is provided as a component separate from the belt unit  64 . However, the cleaning device  32  may be configured to be a part of the belt unit  64 . In  FIG. 5B  (and  FIGS. 5A and 5C  also), the cleaning device  32  is omitted. 
   As described above, the image forming apparatus of the present invention includes at least one image bearing member on which a visible image is formed, an image forming device configured to form the visible image on the at least one image bearing member, an intermediary transfer member onto which visible images of different colors formed on the at least image bearing member are transferred being superimposed one upon another to be formed as a first image, and an intermediary image bearing member onto which the first image on the intermediary transfer member is transferred. The first image transferred onto the intermediary image bearing member is transferred onto one side of a recording medium, and a second image formed on the intermediary transfer member by visible images transferred from the at least one image bearing member being superimposed one upon another is transferred onto the other side of the recording medium. 
   According to the above-described image forming apparatus, a color image can be formed on each side of a recording medium. Further, a color image can be formed on each side of a recording medium by feeding the recording medium only once between the intermediary transfer member and the intermediary image bearing member. Therefore, the time for forming a color image on each side of a recording medium is relatively short, so that productivity of the image forming apparatus is enhanced. Furthermore, instead of causing a recording medium to be born by an intermediary transfer member to pass a plurality of image bearing members so that visible image are transferred onto the recording medium in succession like a background image forming apparatus, a first image formed on the intermediary transfer member by transferring a plurality of visible images formed on the plurality of image bearing members superimposing one upon another is transferred onto the intermediary image bearing member, a second image is formed on the intermediary transfer member by another plurality of visible images, and the first image and the second image are collectively transferred onto one side and the other side of a recording medium, so that misalignment of colors in each image formed on the recording medium can be prevented from occurring or effectively suppressed. 
   In the image forming apparatus configured as illustrated in  FIG. 1 , the intermediary image bearing member  15  must have a surface size suitable to hold at least one page of the first image. If a first image having a size larger than the circumferential length of the intermediary image bearing member  15  is transferred from the intermediary transfer member  3  onto the intermediary image bearing member  15 , when a tip end of the first image transferred onto the intermediary image bearing member  15  reaches a transfer position where the first image is started to be transferred from the intermediary transfer member  3  onto the intermediary image bearing member  15 , transfer of the entire part of the first image from the intermediary transfer member  3  onto the intermediary image bearing member  15  has not been completed yet, so that the first image is transferred onto the intermediary image bearing member  15  in an overlapped state. Conversely, if the circumferential length of the intermediary image bearing member  15  is excessively larger than a size of a recording medium, transfer of the first image from the intermediary transfer member  3  onto the intermediary image bearing member  15  has been completed long before the tip end of the first image transferred on the intermediary image bearing member  15  reaches the above-described transfer position, so that useless time in which a transfer operation is not performed occurs, decreasing image forming efficiency of the image forming apparatus. 
   As described earlier, demands of users are diverse. Some users form an image on each side of a large recording medium, and it is sufficient for other users to form an image on each side of a small recording medium. Accordingly, it is necessary that the surface size of the intermediary image bearing member  15  is set to a size meeting diverse demands of users. 
   Accordingly, the image forming apparatus of the present invention is configured such that either of at least two intermediary image bearing members different from each other in size can be installed in the main body housing  1  in a detachable manner, so that the intermediary image bearing members different in size can be freely exchanged with each other in the image forming apparatus. Here, intermediary image bearing members being different in size includes three cases, one case in which widths of the intermediary image bearing members in a direction perpendicular to surface moving directions of the intermediary image bearing members are different from each other, another case in which circumferential lengths of the intermediary image bearing members are different from each other, and still another case in which both of widths of the intermediary image bearing members in a direction perpendicular to surface moving directions of the intermediary image bearing members and circumferential lengths of the intermediary image bearing members are different from each other. 
     FIG. 6  illustrates an image forming apparatus in which a belt unit  64 A having an intermediary image bearing member  15 A, which is longer in circumferential length than and the same in width as the intermediary image bearing member  15  of  FIG. 1  is used. In the belt unit  64 A illustrated in  FIG. 6 , two supporting rollers  70 A and  71 A are arranged, instead of the supporting rollers  17  and  18  of the belt unit  64  illustrated in  FIG. 1 , to support the intermediary image bearing member  15 A, and a backing member  72 A is added exclusively for the cleaning blade  54  of the cleaning device  32 . Further, a cover  66 A covering components of the belt unit  64 A is formed large enough to accommodate the intermediary image bearing member  15 A that is longer in circumferential length than the intermediary bearing member  15  of  FIG. 1 , so that the cover  66 A is larger than the cover  66  of the belt unit  64  illustrated in  FIG. 1 . That is, the supporting rollers  70 A and  71 A are provided to support the intermediary image bearing member  15 A which is longer than the intermediary image bearing member  15  of  FIG. 1 . The backing member  72 A illustrated in  FIG. 6  is constituted by a roller, and respective axes of the backing member  72 A and the supporting rollers  70 A and  71 A are supported by both side plates of the belt unit  64 A (corresponding to the side plates  65  illustrated in  FIG. 4 ) at end parts thereof so as to be freely rotatable. The side plates of the belt unit  64 A are also formed larger than the side plates  65  illustrated in  FIG. 4  to be able to accommodate the intermediary image bearing member  15 A that is longer in circumferential length than the intermediary image bearing member  15 . Configuration of the other parts of the belt unit  64 A is substantially the same as that of the belt unit  64  illustrated in  FIG. 1 , and respective components in  FIG. 6  corresponding to those of  FIG. 1  are denoted by the same references as those used in  FIG. 1 , respectively appended by A. An exemplary outer appearance of the belt unit  64 A is illustrated in  FIG. 5A . 
   The image forming apparatus of  FIG. 6  can form an image on one side or each side of a recording medium as in the image forming apparatus of  FIG. 1 . However, because the intermediary image bearing member  15 A that is longer in circumferential length than the intermediary image bearing member  15  of the image forming apparatus of  FIG. 1  is used in the image forming apparatus of  FIG. 6 , a user can form a color image on each side of a larger recording medium with the image forming apparatus of  FIG. 6  than with the image forming apparatus of  FIG. 1 . Conversely, a user who forms an image mostly on a recording medium that is smaller in size can form a color images on each side of the recording medium in an efficient manner with the image forming apparatus of  FIG. 1 . 
   A belt unit  64 B illustrated in  FIG. 5C  includes an intermediary image bearing member smaller in width than the intermediary image bearing members  15  and  15 A used in the belt units  64  and  64 A. Configuration of the other parts of the belt unit  64 B is substantially the same as that of the belt unit  64  illustrated in  FIG. 5B , and respective components of the belt unit  64 B in  FIG. 5C  corresponding to those of the belt unit  64  in  FIG. 5B  are denoted by the same references as those used in  FIG. 5B , respectively appended by B. In  FIGS. 5A ,  5 B and SC, W 1  and W 2  indicate widths of respective image bearing members, and the width W 2  is smaller than the width W 1 . Those users who mostly form an image on one or each side of a recording medium small in width, such as a post card, may install the belt unit  64 B including the intermediary image bearing member that is smaller in width in the image forming apparatus. The belt unit  64 B is less expensive than the belt units  64  and  64 A because of the intermediary image bearing member being smaller in width, and thereby the financial burden on the user can be decreased. 
   Now, a concrete example of an installing mechanism for installing any of a plurality of intermediary image bearing members that are different in size in the main body housing  1  in a detachable manner, so that an installed intermediary image bearing member can be exchanged with another one, is described. 
   The main body housing  1  of the image forming apparatus illustrated in  FIG. 1  and  FIG. 6  includes a fixed housing  1 A, and a movable housing  1 B supported by the fixed housing  1 A to open and close relative to the fixed housing  1 A. The movable housing  13  is configured to receive and guide either of the intermediary image bearing members  15  or  15 A to be installed in the installing part of the main body housing  1 . By putting the movable housing  1 B in an opened position, either the intermediary image bearing members  15  or  15 A can be received by and taken out from the movable housing  1 B, respectively. More specifically, the movable housing  1 B is supported by the fixed housing  1 A via a supporting axis  35  so as to rotate to open and close, and putting the movable housing  1 B in the opened position, either of the belt units  64  or  64 A, including the respective intermediary image bearing members  15  or  15 A, can be received by and taken out from the movable housing  1 B. 
     FIG. 7  illustrates a state in which the movable housing  1 B illustrated in  FIG. 1  has been rotated in the direction of an arrow F around the supporting axis  3  so that the movable housing  1 B has been brought into the opened position and thereby the belt unit  64  has been removed from the installing part of the main body housing  1 . The installing part of the main body housing  1  is denoted by a reference symbol IP. The installing part IP is configured such that any of the belt units  64  and  64 A including the respective intermediary image bearing members  15  or  15 A or the belt unit  64 B including the intermediary image bearing member that is smaller in width than the intermediary image bearing members  15  and  15 A can be installed therein. When the movable housing  1 B is opened as illustrated in  FIG. 7 , the area above the movable housing  1 B is opened, so that the belt unit  64  received by the movable housing  1 B can be lifted up to be taken out from the movable cover  1 B. The belt unit  64 A including the intermediary image bearing member  15 A that is long in circumferential length and the belt unit  64 B including the intermediary image bearing member that is smaller in width can be also removed from the installing part IP of the main body housing land taken out from the movable housing  1  in substantially the same manner as in the belt unit  64 . In the image forming apparatus of this example, the cleaning device  32  that is separate from the belt units  64 ,  64 A and  64 B is also supported by the movable housing  1 B, so that the cleaning device  32  is also rotated together with the movable housing  1 B.  FIG. 8  and  FIG. 9  are cross sections illustrating the belt units  64  and  64 A removed from the installing part IP of the main body housing  1  and taken out from the movable housing  1 B, respectively. 
   The movable housing  1 B is configured such that any of the belt units  64 ,  64 A and  64 B can be received. Therefore, for example, when a user using the image forming apparatus of  FIG. 1 , in which the intermediary image bearing member  15  that is short in circumferential length is installed and which is therefore relatively inexpensive, is necessitated to obtain an image forming apparatus capable of forming a color image on each side of a recording medium that is larger in size, the user can configure the image forming apparatus of  FIG. 1  to the image forming apparatus illustrated in  FIG. 6  by simply obtaining the belt unit  64 A having the intermediary image bearing member  15 A that is long in circumferential length. The user opens the movable housing  1 B, thereby removing the belt unit  64  from the installing part IP of the main body housing  1 , as illustrated in  FIG. 7 , then, takes out the belt unit  64  from the movable housing  1 B, causes the newly obtained belt unit  64 A to be received by the movable housing  1 B, and closes the movable housing  1 B, and thereby the intermediary image bearing member  15 A long in circumferential length is installed in the installing part IP of the main body housing  1  of the image forming apparatus of  FIG. 1 . Thereby, the image forming apparatus of  FIG. 6  that can form an image on a larger recording medium is obtained. Thus, in the image forming apparatus of the present invention, any of the belt units  64 ,  64 A and  64 B which has been previously installed in the main body housing  1  can be exchanged with any one of the belt units  64 ,  64 A and  64 B, so that, according to the present invention, an image forming apparatus capable of forming a color image on each side of a recording medium having a size desired by the user can be freely realized by exchanging a part thereof with another one. 
   Further, when manufacturing image forming apparatuses, by causing either of the belt units  64 ,  64 A and  64 B to be received by the movable housing  1 B of each image forming apparatus, image forming apparatuses having different functions or performances can be manufactured, respectively. Thereby, the need to meet diverse needs of users can be easily realized. 
     FIG. 5D  illustrates a state of the image forming apparatus illustrated in  FIG. 1  and  FIG. 6 , in which the movable housing  1 B is opened and thereby the belt unit  64 ,  64 A or  64 B has been removed from the installing part IP of the main body housing  1  and the belt unit  64 ,  64 A or  64 B has been taken out from the movable housing  1 B. In contrast,  FIG. 10  illustrates image forming apparatuses  100 ,  100 A and  100 B, each in a state that the movable housing  1 B is closed. The belt unit  64  or  64 B is installed in the image forming apparatus denoted by reference numeral  100 , the belt unit  64 A is installed in the image forming apparatus denoted by reference numeral  100 A, and no belt unit is installed in the image forming apparatus denoted by reference numeral  100 B. 
   As illustrated in  FIG. 5D  and  FIG. 10 , a lock releasing lever  37  is provided to the movable housing  1 B. When the movable housing  1 B is closed, the movable housing  1 B is locked to the fixed housing  1 A, and in a state that the movable housing  1 B is closed, by operating the lock releasing lever  37 , the movable housing  1 B is released from being locked to the fixed housing  1 A, and thereby the movable housing  1 B can be rotated to the opened position illustrated in  FIG. 5D . An opening  38  is formed in the movable housing  1 B such that each of the belt unit  64 ,  64 A and  64 B including intermediary image bearing member  15  or  15 A different in size can be installed in the installing part P of the main body housing  1  as described later. 
   Further, as illustrated in  FIGS. 5A ,  5 B and  5 C, knobs  39 A,  39  and  39 B are provided to the covers  66 A,  66  and  66 B of the belt units  64 A,  64  and  64 B, respectively, so that the operator can install and remove the belt units  64 A,  64  and  64 B in and from the installing part IP of the main body housing  1 , in this case, via the movable housing  1 B, by grasping the knob  39 A,  39  and  39 B, respectively. The positions of the knobs  39 A,  39  and  39 B relative to the covers  66 A,  66  and  66 B are set such that when the movable housing  1 B in which the belt unit  64 A,  64  or  64 B has been received is closed, the knob  39 A,  39  or  39 B is concealed inside of the movable housing  1 B as can be understood from the view of  FIG. 10 , and when the movable housing  1 B is opened, the knob  39 A,  39  or  39 B is exposed. Thereby, when the image forming apparatus is operating with the movable housing  1 B closed, the belt unit  64 A,  64  or  64 B cannot be removed from the main body housing  1 , because the operator cannot grasp the knob  39 A,  39  or  39 B of the belt unit  64 A,  64  or  64 B, so that the operator cannot remove the belt unit  64 A,  64  or  64 B from the main body housing  1 . Thereby, when electric current is being supplied to the image forming apparatus so that the image forming apparatus is in an operating condition, it will never occur that the belt unit  64 A,  64  or  64 B is inadvertently removed from the apparatus. When the movable housing  1 B is opened, a switch (not shown) is turned off, so that current distribution to the image forming apparatus is shut off. In that state, the operator can safely take out the belt unit  64 A,  64  or  64 B removed from the installing part IP of the main body housing  1  from the movable housing  1 B by grasping the exposed knob  39 A,  39  or  39 B. 
   More specifically, as illustrated in  FIG. 5D , guiding grooves  40 , serving as an example of a receiving and guiding mechanism configured to receive and to guide any of the belt units  64 A,  64  and  64 B to be installed in the installing part IP of the main body housing  1 , are formed on internal surfaces of side walls of the movable housing  1 B. Further, as illustrated in  FIG. 5A ,  5 B and  5 C, guiding protrusions  41 A,  41  and  41 B configured to engage with the guiding grooves  40  of the movable housing  1 B are provided to the covers  66 A,  66  and  66 B of the belt units  64 A,  64  and  64 B, respectively. By engaging the guiding protrusions  41 A,  41  or  41 B of the belt unit  64 A,  64  or  64 B with the guiding grooves  40  of the movable housing  1 B, the belt unit  64 A,  64  or  64 B is received by the movable housing  1 B. Thus, each of the belt units  64 ,  64 A and  64 B can be easily received by and taken out from the movable housing  1 B. 
   Furthermore, as illustrated in  FIG. 1 ,  FIG. 6  and  FIG. 7 , a locking member  42 , e.g., a spring, is arranged to the fixed housing  1 A, and as illustrated in  FIGS. 5A ,  5 B and  5 C, locking grooves  43 A,  43  and  43 B with which the locking member  42  is engaged are formed on the covers  66 A,  66  and  66 B, respectively. When the movable housing  1 B is closed as illustrated in  FIG. 1  and  FIG. 6  in a state that the belt unit  64 A,  64  or  64 B is received by the movable housing  1 B, the locking member  42  engages with the locking groove  43 A,  43  or  43 B, thereby pressing the cover  66 A,  66  or  66 B downward so that the belt unit  64 A,  64  or  64 B is positioned in a predetermined position in the installing part IP of the main body housing  1 . At this time, the movable housing  1 B is locked relative to the fixed housing  1 A, gear  68 A,  68  or  68 B of the belt unit  64 A,  64  or  64 B engages with the driving gear  69  supported by the fixed housing  1 A, and the transfer roller  20 A,  20  or  20 B of the belt unit  64 A,  64  or  64 B is brought into contact with the output contact point of the power source as describe earlier. Thus, the belt unit  64 A,  64  or  64 B received by the movable housing  1 B is installed in the installing part IP of the main body housing  1  by closing the movable housing  1 B, and thereby the intermediary image bearing member  15  or  15 A included in the belt unit  64 A,  64  or  64 B is installed in the installing part IP of the main body housing  1 . 
   The movable housing  1 B may be supported by the fixed housing  1 A so as to move in a horizontal direction, instead of being rotatably supported by the fixed housing  1 A, to be released from the fixed housing  1 A, as illustrated in  FIG. 11 , for example, using a rail member or a linking mechanism, not shown. In this case also, after bringing the movable housing  1 B into an opened position, by engaging the guiding protrusions  41 A,  41  or  41 B of the belt unit  64 A,  64  or  64 B with the guiding grooves  40  of the movable housing  1 B, the belt unit  64 A,  64  or  64 B including the intermediary image bearing member  15  or  15 A can be received by and taken out from the movable housing  1 B. Also, in the image forming apparatus of  FIG. 11 , the cleaning device  32  is supported by the movable housing  1 B. In this example, because the movable housing  1 B is moved in a horizontal direction, an advantage is obtained because used toner collected in the case  67  of the cleaning device  32  is difficult to spill from the case  67 . 
   In each of the image forming apparatuses illustrated in  FIG. 7  and  FIG. 11 , the movable housing  1 B is supported by the fixed housing  1 A such that when the movable housing  1 B is brought into the opened position, the intermediary image bearing member  15  of the belt unit  64  received by the movable housing  1 B is separated from the intermediary transfer member  3  mounted to the fixed housing  1 A. Therefore, when the belt unit  64  is received by and taken out from the movable housing  1 B, the intermediary image bearing member  15  never touches the intermediary transfer member  3 , so that damage to the intermediary image bearing member  15  or the intermediary transfer member  3  is prevented. Further, when the movable housing  1 B is opened, a conveying path of a recording medium is largely opened, so that a recording medium jammed in the conveying path can be easily cleared. Thus, the movable housing  1 B can be opened for other purposes than for exchanging the belt unit  64  with another one to facilitate performance of necessary operations such as clearing jams. 
   In each of the image forming apparatuses illustrated in  FIG. 1 ,  FIG. 5D ,  FIG. 6  and  FIG. 7 , the movable housing  1 B is supported by the fixed housing  1 A so as to rotate between the closed position and the opened position. However, the movable housing  1 B can be configured to further rotate, exceeding the opened position illustrated in  FIG. 7 , to another opened position illustrated in  FIG. 12 , and to be supported at each position. That is, the movable housing  1 B is configured to be supported by the fixed housing  1 A at two opened positions, a first opened position where the movable housing  1 B reaches after rotating a relatively small angle from the closed position, e.g., the opened position of the movable housing  1 B in  FIG. 7 , and a second opened position where the movable housing  1 B reaches after rotating a greater angle from the closed position, e.g., the opened position of the movable housing  1 B in  FIG. 11 . The movable housing  1 B may be supported, for example, by a stopper (not shown) at the first and second opened positions, respectively. 
   According to the above-described configuration, a belt unit is received by and taken out from the movable housing  1 B in a state that the movable housing  1 B is opened to the first opened position, and when a recording medium is jammed, the movable housing  1 B may be opened to the second opened position so that the conveying path of the recording medium is greatly opened, and thereby the jammed recording medium can be easily cleared and the inside of the main body housing  1  can be easily cleaned. At that time, as illustrated in  FIG. 11  and  FIG. 12 , because a roller  26 A of the registration roller pair  26  is supported by the fixed housing  1 A and a roller  26 B of the registration roller pair  26  is supported by the movable housing  1 B, by rotating the movable housing  1 B to the second opened position, the rollers  26 A and  26 B of the registration roller pair  26  are greatly separated from each other, so that removal of a recording medium jammed between the rollers  26 A and  26 B and cleaning of the rollers  26 A and  26 B can be easily performed. Further, because the corona discharging device  27  is supported by the fixed housing  1 A, by opening the movable housing  1 B to the second opened position, the corona discharging device  27  can be easily accessed, so that cleaning thereof can be easily performed. 
   Further, in the image forming apparatus of the present invention, the intermediary image bearing members  15  and  15 A are constituted of seamless belts driven to rotate, respectively, regardless of their sizes. Therefore, each of the intermediary image bearing members  15  and  15 A, regardless of whether it is short or long in circumferential length, can be configured to be compact to be easily accommodated in the installing part IP of the main body housing  1 , so that the main body housing  1  can be prevented from being made large. 
   Furthermore, the image forming apparatus is configured, as described above, such that at least the intermediary image bearing members  15  and  15 A, and the plurality of belt supporting rollers  16 ,  17 ,  18 ,  19  and the plurality of belt supporting rollers  16 A,  19 A,  70 A,  71 A, around which the intermediary image bearing members  15  and  15 A are spanned, respectively, are integrated with each other to be formed as the belt units  64 A,  64  and  64 B, respectively. Further, the installing mechanism for installing any of the intermediary image bearing members  15  or  15 A different in size in the installing part IP of the image forming apparatus is configured such that by installing any of the belt units  64 ,  64 A or  64 B having the intermediary image bearing members  15  or  15 A different in size in the installing part IP in a detachable manner, the intermediary image bearing member  15  or  15 A is installed in the installing part IP of the main body housing  1  of the image forming apparatus, so that a desired intermediary image bearing member can be easily installed in an image forming apparatus and thereby the image forming apparatus meeting a need of a user can be easily configured. 
   Further, in the image forming apparatus of the present invention, the belt units  64 ,  64 A and  64 B including the intermediary image bearing members  15  and  15 A that are different in size are configured such that at least some parts of the belt units  64 ,  64 A and  64 B are installed in substantially the same positions in the main body housing  1  of the image forming apparatus, respectively. Thereby, control of an image forming operation of the image forming apparatus is prevented from being complicated. 
   Exemplary configurations of the belt units  64  and  64 A achieving the above-described feature of the present invention will be described referring to  FIG. 1  and  FIG. 6 , however, the belt unit  64 B can also be configured in a similar manner. 
   The belt units  64  and  64 A having the intermediary bearing members  15  and  15 A different from each other in size include the transfer rollers  20  and  20 A as transfer devices performing a function of transferring the first image on the intermediary transfer member  3  onto the intermediary image bearing members  15  and  15 A as well as a function of transferring the second image on the intermediary transfer member  3  onto the other side of a recording medium P. The belt unit  64  and  64 A are configured such that the transfer rollers  20  and  20 A of the belt units  64  and  64 A having the intermediary image bearing members  15  and  15 A that are different from each other in size are installed substantially in the same position in the main body housing  1 , respectively. With this configuration, transfer conditions when transferring the first image on the intermediary transfer member  3  onto the intermediary image bearing member  15  or  15 A and when transferring the second image on the intermediary transfer member  3  onto the other side of the recording medium P can be made substantially the same when the intermediary image bearing member  15  that is short in circumferential length is installed in the installing part IP of the main body housing  1  and when the intermediary image bearing member  15 A that is longer in circumferential length than the intermediary image bearing member  15  is installed in the installing part IP of the main body housing  1 . Thereby, control of the image forming operation of the image forming apparatus can be prevented from being complicated. 
   Further, in the belt units  64  and  64 A, recording medium conveying paths extending substantially in a vertical direction are formed by the intermediary image bearing members  15  and  16 A constituted by seamless belts such that a recording medium P is conveyed to transfer positions in the recording medium conveying paths, where the second and first images are respectively transferred onto the recording medium P, from positions below the transfer positions, respectively. Here, the belt units  64  and  64 A including the intermediary image bearing members  15  and  15 A that are different from each other in size are configured such that the supporting rollers  19  and  16 , and  19 A and  16 A, supporting parts of the intermediary image bearing members  15  and  15 A, forming the recording medium conveying paths of the belt units  64  and  64 A, are installed in substantially the same position in the main body housing  1 , respectively. With this configuration, recording medium conveying conditions in the recording medium conveying paths formed by the intermediary image bearing members  15  and  15 A of the belt units  64  and  64 A can be made substantially the same when the intermediary image bearing member  15  that is short in circumferential length is installed in the installing part IP of the main body housing  1  and when the intermediary image bearing member  15 A that is longer in circumferential length than the intermediary image bearing member  15  is installed in the installing part IP of the main body housing  1 . Thereby, in the image forming apparatus of the present invention, it is not necessary to change a recording medium conveying condition each time when an intermediary image bearing member of one size is exchanged with another one of a different size, so that control of an image forming operation can be prevented from being complicated. 
   Furthermore, the belt units  64  and  64 A include the opposing electrodes  63  and  63 A arranged to oppose a transfer device transferring the first image transferred onto the intermediary image bearing members  15  and  15 A onto one side of a recording medium P (i.e., the corona discharging device  27  in each of the illustrated examples) sandwiching the intermediary image bearing members  15  and  15 A with the transfer device and the opposing electrodes  63  and  63 A, respectively. Here, the belt units  64  and  64 A including the intermediary image bearing members  15  and  15 A that are different from each other in size are configured such that the opposing electrodes  63  and  63 A of the belt units  64  and  64 A are installed in substantially the same position in the main body housing  1 , respectively. With this configuration, transferring conditions for transferring the first image onto one side of the recording medium P from the intermediary image bearing members  15  and  15 A can be made substantially the same when the intermediary image bearing member  15  that is short in circumferential length is installed in the installing part IP of the main body housing  1  and when the intermediary image bearing member  15 A that is longer in circumferential length than the intermediary image bearing member  15  is installed in the installing part IP of the main body housing  1 , so that control of an image forming operation of the image forming apparatus can be prevented from being complicated. 
   The cleaning devices  32  for cleaning the intermediary image bearing members  15  and  15 A after transferring the first image onto one side of a recording member P are configured, as illustrated in  FIG. 1  and  FIG. 6 , separately from the belt units  64  and  64 A, respectively. However, as described earlier, the cleaning devices  32  can be configured as components constituting the belt units  64  and  64 A, respectively. When the belt units  64  and  64 A are thus configured to include the cleaning devices  32 , respectively, the belt units  64  and  64 A including the intermediary image bearing members  15  and  15 A that are different from each other in size may be configured such that the cleaning devices  32  of the belt units  64  and  64 A are installed in substantially the same position in the main body housing  1 , respectively. By adopting the above-described configuration, cleaning conditions for the intermediary image bearing members  15  and  15 A can be made substantially the same when the intermediary image bearing member  15  that is short in circumferential length is installed in the installing part IP of the main body housing  1  and when the intermediary image bearing member  15 A that is longer in circumferential length than the intermediary image bearing member  15  is installed in the installing part IP of the main body housing  1 . 
   The belt units  64  and  64 A include the backing members for the cleaning members of the cleaning devices  32  (i.e., the cleaning blades  54  in the illustrated examples)for cleaning surfaces of the intermediary image bearing members  15  and  15 A of seamless belts after transferring the first image onto one side of a recording medium P, arranged to oppose the cleaning blades  54  of the cleaning devices  32  sandwiching the intermediary image bearing members  15  and  15 A with the backing members and the cleaning members of the cleaning devices  32 , respectively. In the belt unit  64  illustrated in  FIG. 1 , the supporting roller  18  serves as the backing member, and in the belt unit  64 A illustrated in  FIG. 6 , an exclusive backing member  72 A is provided. Here, the belt units  64  and  64 A having the intermediary image bearing members  15  and  15 A that are different from each other in size are configured such that the backing members of the belt units  64  and  64 A are installed in substantially the same position in the main body housing  1 , respectively. With the above-described configuration also, cleaning conditions for the intermediary image bearing members  15  and  15 A can be made substantially the same when the intermediary image bearing member  15  that is short in circumferential length is installed in the installing part IP of the main body housing  1  and when the intermediary image bearing member  15 A that is longer than the intermediary image bearing member in circumferential length is installed in the installing part IP of the main body housing  1 . 
   Further, as described above, the screws  68  and  68 A are fixed to the supporting rollers  16  and  16 A of the belt units  64  and  64 A, and when the belt units  64  and  64 A are installed in the installing part IP of the main body housing  1 , the screws  68  and  68 A engage with the driving gear  69  provided to the main body housing  1 , respectively. The supporting rollers  16  and  16 A are rotated via the screws  68  and  68 A, and thereby the intermediary image bearing members  15  and  15 A and other supporting rollers are driven. Thus, the supporting rollers  16  and  16 A serve as driving rollers. By configuring the belt units  64  and  64 A such that the supporting rollers  16  and  16 A are installed in substantially the same position in the main body housing  1 , respectively, the screws  68  and  68 A fixed to the supporting rollers  16  and  16 A engage with the driving gear  69  provided to the main body housing  1 , so that the intermediary image bearing members  15  and  15 A are driven to rotate without any problem, when the belt unit  64  having the intermediary image bearing member  15  that is short in circumferential length is installed in the installing part IP of the main body housing  1  and when the belt unit  64 A having the intermediary image bearing member  15 A that is longer in circumferential length than the intermediary image bearing member  15  is installed in the installing part IP of the main body housing  1 . That is, the belt units  64  and  64 A including the intermediary image bearing members  15  and  15 A that are different from each other in size are configured such that those of the supporting rollers supporting the intermediary image bearing members  15  and  15 A, serving as driving rollers, are installed in substantially the same position in the main body housing  1 , respectively. 
   Thus, by configuring the belt units  64 ,  64 A and  64 B such that at least some parts of the belt units  64 ,  64 A and  64 B are installed in substantially the same positions in the main body housing  1 , respectively, the above-described various advantages can be obtained. Further, in manufacturing, some parts of the belt units  64 ,  64 A and  64 B can be made common parts, so that production costs of the belt units  64 ,  64 A and  64 B can be reduced. 
   As can be understood by comparing the cross section of  FIG. 1  with that of  FIG. 6 , the intermediary image bearing member  15 A that is long in circumferential length is installed in the installing part IP of the main body housing  1  of the image forming apparatus in such a manner that the intermediary image bearing member  15 A protrudes to the side opposite the intermediary transfer member  3  more than the intermediary image bearing member  15  that is shorter than the intermediary image bearing member  15 A in circumferential length protrudes. Thereby, the parts of the intermediary image bearing members  15  and  15 A opposing the intermediary transfer member  3 , i.e., the parts of the intermediary image bearing members  15  and  15 A forming the recording medium conveying paths, can always be made substantially the same regardless of the circumferential length of the intermediary image bearing members  15  or  15 A installed in the main body housing  1 , so that a recording medium can always be conveyed in the same condition, and thereby control of an image forming operation of the image forming apparatus can be prevented from being complicated. 
   Further, as illustrated in  FIG. 5D  and  FIG. 10 , the opening  38  is formed in the movable housing  1 B of the main body housing  1 , so that when the belt unit  64 A including the intermediary image bearing member  15 A that is long in circumferential length is installed in the installing part IP of the main body housing  1  as in the image forming apparatus  100 A of  FIG. 10 , the cover  66 A of the belt unit  64 A protrudes from an adjacent part of the main body housing  1 . That is, the belt units  64 A,  64  and  64 B have the covers  66 A,  66  and  66 B covering respective components of the belt units  64 A,  64  and  64 B, as illustrated in  FIG. 5A ,  5 B and  5 C, and the opening  38  is formed in the movable housing  1 B such that the covers  66 A,  66  and  66 B are allowed to protrude from parts of the main body housing  1  adjacent to the covers  66 A,  66  and  66 B, respectively. With this configuration, any of the belt units  64 A,  64  and  64 B can be installed in the installing part IP of the main body housing  1  without changing the main body housing  1 , or only by slightly changing the main body housing  1 . Thereby, when an installed belt unit is exchanged with another one, additional increases in cost are prevented. 
   Furthermore, as can be understood from  FIG. 10 , exposed parts of the covers  66 A,  66  and  66 B of the belt units  64 A,  64  and  64 B are configured to serve as exterior parts of the image forming apparatuses, respectively. With this configuration, whichever of the belt units  64 A,  64  and  64 B is installed in the main body housing  1 , it is not necessary to provide a cover to cover the installed one of the belt unit  64 A,  64  and  64 B, thus preventing additional increases in cost. 
   Further, as described earlier referring to  FIG. 1 , in the image forming apparatus of the present invention, the sheet feeding cassette  24  accommodating the recording medium P is installed to be drawn out in the direction of the arrow E relative to the main body housing  1 . The position of the opening  38  is formed such that the cover  66 A of the belt unit  64 A, for example, protrudes in the direction of the arrow E in which the sheet feeding cassette  24  is drawn out. 
   By adopting the above-described configuration, even when the belt unit  64 A relatively large in size is installed in the main body housing  1 , the overall height of the image forming apparatus including the belt unit  64 A can be prevented from being increased compared with a case that the belt unit  64  or  64 B that is relatively small in size is installed in the main body housing  1 . Furthermore, when installing the image forming apparatus, a space must be secured in the direction of the arrow E in which the sheet feeding cassette  24  is drawn out. When the belt unit  64 A that is relatively large in size is installed in the main body housing  1 , because the belt unit  64 A protrudes toward the side of the above-described space that is required for drawing out the sheet feeding cassette  24 , the installing space for the image forming apparatus will not be further increased. That is, the installation spaces of the image forming apparatus of this example when the belt unit  64 A that is relatively large in size is installed in the main body housing  1  and when the belt unit  64  that is relatively small in size is installed in the main body housing  1  can be the same. 
   Further, in the image forming apparatus of the present invention, a size recognition device for recognizing the size of an intermediary image bearing member installed in the main body housing  1  of the image forming apparatus is provided. For example, as illustrated in  FIGS. 5A ,  5 B and  5 C, recognizing parts MA, M and MB, constituted by protrusions in this example, are provided to the covers  66 A,  66  and  66 B of the belt units  64 A,  64  and  64 B, respectively. The recognizing parts MA, M and MB are arranged in such positions of the belt units  64 A,  64  and  64 B that when the belt units  64 A,  64  and  64 B are installed in the installing part IP of the main body housing  1 , the recognizing parts MA, M and MB are located in positions different from each other in the main body housing  1 . The recognizing parts MA, M and MB are detected by detecting devices such as photo-sensors and micro-switches (not shown) provided in the main body housing  1 , respectively, and thereby which of the belt units  64 A,  64  and  64 B is installed in the installing part IP of the main body housing  1  can be automatically determined. Thereby, control corresponding to each intermediary image bearing member can be performed in forming an image. 
   Furthermore, by providing marks, labels or descriptions indicating the sizes of respective intermediary image bearing members included in the belt units  64 A,  64  and  64 B at parts of the covers  66 A,  66  and  66 B, that can be seen from outside when the belt units  64 A,  64  and  64 B are installed in the installing part IP of the main body housing  1 , respectively, a user using the image forming apparatus can surely recognize the maximum size of a recording medium on which an image can be formed with the image forming apparatus. Thereby, the user can operate the image forming apparatus without being uncertain about the maximum size of a recording medium on which an image can be formed. 
   Further, as illustrated in  FIG. 13  and  FIG. 14 , the movable housing  1 B may be configured to include a first cover member  44 A rotatably supported by the fixed housing  1 A via the supporting axis  35  ( FIG. 14 ) and a second cover member  44 B rotatably connected with the first cover member  44 A via another supporting axis  35 A. The supporting axis  35  and the another supporting axis  35 A are parallel to each other, and the first cover member  44 A rotates in the clockwise direction around the supporting axis  35  and the second cover member  44 B rotates in the counterclockwise direction around the another supporting axis  35 A in  FIG. 13  and  FIG. 14 , so that the movable cover  1 B bends at the position of the another supporting axis  35 A. After opening the movable cover  1 B as illustrated in  FIG. 13  and  FIG. 14 , the belt units  64 ,  64 A and  64 B can be received by and taken out from the movable cover  1 B, respectively. 
   As illustrated in  FIG. 14 , a connecting member  45  is rotatably attached to the fixed housing  1 A and the second cover member  44 B at both ends thereof in the longitudinal direction thereof via pins  46  and  47 , respectively, and one end of a regulating member  48  is rotatably connected with the second cover member  44 B via a pin  49 . A guide pin  50  provided to the other end of the regulating member  48  is engaged with a long hole  51  formed in the connecting member  45  to slide therein. 
   When the movable housing  1 B is put in the opened position as illustrated in  FIG. 14 , the guide pin  50  of the regulating member  49  contacts a lower end of the long hole  51 , so that the movable housing  1 B is prevented from being further rotated. Thereby, the movable housing  1 B is kept in that opened position. As the movable housing  1 B is closed, the guide pin  50  slides toward an upper end of the long hole  51 , and when the movable housing  1 B is completely closed, the first and the second cover members  44 A and  44 B are put in a state that surfaces thereof are flat in the vertical direction. 
   The connecting member  45  and the regulating member  48  can be provided at both sides of the movable housing  1 B. However, in this example, these members are only provided at the rear side of the apparatus, so that workability from the front side of the apparatus is not hindered, which is advantageous. 
   The movable housing  1 B may be configured by three or more cover members such that the movable housing  1 B is bent at respective connecting parts thereof. Thus, by configuring the movable housing  1 B by at least two cover members connected with each other such that the movable cover  1 B is bent at respective connecting parts thereof when the movable cover  1 B is put in an opened position, the space occupied by the movable housing  1 B when the movable housing  1 B is opened can be minimized, so that the image forming apparatus can be installed in a relatively small place. 
   In each of the image forming apparatuses described above, the corona discharging device  27 , an example of the transfer device transferring the first image on the intermediary image bearing member  15  or  15 A onto one side of a recording medium P, is arranged separately from the belt unit  64  or  64 A. However, the corona discharging device  27  can be a part of the belt units  64  and  64 A. That is, the belt units  64  and  64 A can be configured to include a transfer device for transferring the first image on the intermediary image bearing members  15  and  15  A of endless belts onto one side of a recording medium P, respectively. 
   In this case, although the corona discharging devices  27  as the transfer devices for transferring the first image on the intermediary image bearing members  15  and  15 A of endless belts onto one side of a recording medium P are arranged outside of the intermediary image bearing members  15  and  15 A, respectively, in the image forming apparatuses illustrated in  FIG. 1  and  FIG. 6 , the transfer devices for transferring the first image on the intermediary image bearing members  15  and  15 A of endless belts onto one side of a recording medium P can be arranged inside of the intermediary image bearing members  15  and  15 A, respectively. 
     FIG. 15  and  FIG. 16  illustrate exemplary configurations of the image forming apparatus in which a transfer device for transferring the first image on the intermediary image bearing member  15  or  15 A onto one side of a recording medium is arranged inside of the intermediary image bearing member  15  or  15 A, respectively.  FIG. 15  illustrates an exemplary configuration of the belt unit  64  including the intermediary image bearing member  15  of an endless belt that is short in circumferential length and the supporting rollers  75 ,  76 ,  77  and  78  around which the intermediary image bearing member  15  is spanned.  FIG. 16  illustrates an exemplary configuration of the belt unit  64 A including the intermediary image bearing member  15 A of an endless belt that is long in circumferential length and the supporting rollers  75 A,  76 A,  77 A and  78 A around which the intermediary image bearing member  15 A is spanned. The intermediary image bearing members  15  and  15 A are driven to rotate in the direction indicated by the arrow B, respectively. In each of the configurations, the intermediary transfer member  3  of an endless belt is driven to rotate in the direction indicated by the arrow A. 
   In each of the configurations, as in the image forming apparatuses described earlier, toner images of different colors are transferred from the image bearing members  2 Y,  2 M,  2 C and  2 BK onto the intermediary transfer member  3  to be superimposed one upon another to form a first image thereupon, the first image is then transferred onto one side of a recording medium (not shown), and a second image formed on the intermediary transfer member  3  by visible images subsequently transferred thereupon to be superimposed one upon another is transferred onto the other side of the recording medium. At this time, in the examples illustrated in  FIG. 15  and  FIG. 16 , the supporting rollers  75  and  75 A function as a transfer device to transfer the first image on the intermediary transfer member  3  onto the intermediary image bearing members  15  and  15 A and to transfer the second image on the intermediary transfer member  3  onto the other side of the recording medium, and the supporting rollers  76  and  76 A function as a transfer device to transfer the first image transferred on the intermediary image bearing members  15  and  15 A on to the one side of the recording medium, respectively. In these examples, a transfer voltage of plus polarity is applied to the supporting rollers  75  and  75 A and a transfer voltage of minus polarity is applied to the supporting rollers  76  and  76 A, respectively, so that respective transfer operations are performed. Thus, a transfer device to transfer the first image on an intermediary image bearing member onto one side of a recording medium is arranged inside of the intermediary image bearing member, and further, the transfer device is constituted by a transfer roller to which a transfer voltage is applied, i.e., in the illustrated examples, the supporting rollers  76  and  76 A. 
   Further, a corona discharging device including a corona wire to which a transfer voltage is applied may be arranged as the transfer device inside of the intermediary image bearing member  15  or  15 A so that the first image on the intermediary image bearing member  15  or  15 A is transferred onto one side of a recording medium. 
   When the belt units  64  and  64 A including the intermediary image bearing members  15  and  15 A that are different in size are configured, as described above, to include transfer devices to transfer the first image on the intermediary image bearing members  15  and  15 A onto one side of a recording medium, the belt units  64  and  64 A can be configured such that the transfer devices are installed in substantially the same position in the main body housing  1 , respectively. With these configurations, whichever of the belt units  64  and  64 A is installed in the installing part IP of the main body housing  1 , the first image can be transferred from the intermediary image bearing member  15  or  15 A onto one side of a recording medium at the same position in the main body housing  1 . 
   Other parts of the image forming apparatus illustrated in  FIG. 15  and  FIG. 16  are substantially the same as those of the previously described image forming apparatus. 
   The surface size of an intermediary image bearing member used in each image forming apparatus described above can be appropriately set, however, it is advantageous to set the surface size of the intermediary image bearing member as described below.  FIG. 17A ,  FIG. 17B  and  FIG. 18  are diagrams illustrating the intermediary image bearing members  15  and  15 A in developed states, and a recording medium P 1  and a recording medium P 2  being conveyed by the intermediary image bearing members  15  and  15 A. In the diagrams, an arrow B1 indicates a direction in which each recording medium is conveyed. In the example illustrated in  FIG. 17A , the intermediary image bearing member  15 A is formed in such a size that the first image to be transferred onto the recording medium P 1  of a standard size, being conveyed with a long side thereof corresponding to the conveying direction of B1, can be accommodated, and in the example illustrated in  FIG. 17B , the intermediary image bearing member  15  is formed in such a size that the first image to be transferred onto the recording medium P 2  half in size of the recording medium P 1  of a standard size, being conveyed with a short side thereof corresponding to the conveying direction of B1, can be accommodated. 
   For example, when the size of the recording medium P 1  is A3, the size of the recording medium P 2  is A4, which is half of A3. Similarly, when the size of the recording medium P 1  is A4, the size of the recording medium P 2  is A5, and when the size of the recording medium P 1  is B3, the size of the recording medium P 2  is B4. When the size of the recording medium P 1  is a double-letter size (11 inches×17 inches), which is a U.S. standard size, the size of the recording medium P 2  is a letter size (8.5 inches×11 inches), half of the double-letter size. 
   More specifically, because the length of the short side of an A4-sized recording medium is 210 mm, the circumferential length of the intermediary image bearing member  15  that is small in circumferential length is set, for example, to about 310 mm so that the first image of A4 size can be easily accommodated. Similarly, because the length of the long side of an A3-sized recording medium is 420 m, the circumferential length of the intermediary image bearing member  15 A that is large in size is set, for example, to about 500 mm so that the first image of A 3  size can be accommodated. 
   By configuring the intermediary image bearing members  15  and  15 A as described above, the widths W 1  of the intermediary image bearing members  15  and  15 A can be made the same. Thereby, components constituting the belt units  64  and  64 A including the intermediary image bearing members  15  and  15 A, e.g., supporting rollers, transfer rollers, opposing electrodes, etc., can be surely made common, so that the manufacturing costs of the belt unit  64  and  64 A can be reduced. 
   Further, when the intermediary image bearing member  15 A having a large size that can accommodate the first image to be transferred onto a recording medium of frequently used A3 size is used as illustrated in  FIG. 17A , if a recording medium of also frequently used A4 size is conveyed with the short side thereof corresponding to the recording medium conveying direction B1 and an image is formed on each side of the recording medium of A4 size, then because the circumferential length of the intermediary image bearing member  15 A is much longer than the short side of the recording medium of A4 size, a long time is required to complete formation of the image on each side of the recording medium of A4 size, so that useless time is generated and thereby the efficiency of image formation is decreased. 
   Accordingly, as illustrated in  FIG. 18 , it is preferable that at least one of the intermediary image bearing members  15  and  15 A, in this example, the intermediary image bearing member  15 A, is configured in such a size that an image to be transferred onto the recording medium P 1  of a standard size, being conveyed with the long side thereof corresponding to the conveying direction of B1, can be accommodated and further at least two pages of the first image, to be transferred in succession, while the intermediary image bearing member  15 A makes one revolution, onto two sheets of the recording medium P 2  half in size of the recording medium P 1  of a standard size, being conveyed with the short side thereof corresponding to the conveying direction of B1, can be accommodated. The two pages of the first image, transferred in succession onto the intermediary image bearing member  15 A, are transferred onto one sides of the two sheets of the recording medium P 2 , conveyed in succession. In this case also, when the size of the recording medium P 1  is A3, the size of the recording medium P 2  is A4, when the size of the recording medium P 1  is A4, the size of the recording medium P 2  is A5, when the size of the recording medium P 1  is B3, the size of the recording medium P 2  is B4, and when the size of the recording medium P 1  is a double-letter size, the size of the recording medium P 2  is a letter-size. 
   More specifically, while the intermediary image bearing member  15 A makes one revolution, two pages of the first image to be transferred onto two sheets of the recording medium P 2  of A4 size are respectively transferred from the intermediary transfer member  3  onto the entire surface of the intermediary image bearing member  15 A, the two sheets of the recording medium P 2  are fed out from the sheet feeding device  23  in succession, and the two pages of the first image on the intermediary image baring member  15 A are successively transferred onto respective one sides of the two sheets of the recording medium P 2 , one page of the first image on each sheet of the recording medium P 2 , and two pages of the second image on the intermediary transfer member  3  are respectively transferred onto the other sides of the two sheets of the recording medium P 2 . With this configuration, when compared with a case in which, when forming images on two sheets of the recording medium P 2  of A4 size, first, the first image formed on the intermediary image bearing member  15 A that is large in size is transferred onto one side of the first sheet of the recording medium P 2  and the second image on the intermediary transfer member  3  is transferred onto the other side of the recording medium P 2 , thereafter, the next first image is transferred from the intermediary transfer member  3  onto the intermediary image bearing member  15 A that is large in size, and the next first image is transferred onto one side of the second sheet of the recording medium P 2 , conveyed a predetermined time after the first sheet of the recording medium P, and the next second image on the intermediary transfer member  3  is transferred onto the other side of the second sheet of the recording medium P 2 , the time of image formation can be reduced and thereby the image forming efficiency can be greatly enhanced. In particular, when forming images successively on a great number of sheets of a recording medium, the image forming efficiency can be enhanced. The above-described advantage can be obtained in a similar manner when the recording medium is in other sizes. 
   In adopting the above-described configuration, when feeding two sheets of an A4-sized recording medium in succession from the sheet feeding device  23 , a slight interval is provided between the two sheets of the A4-sized recording medium. Therefore, taking the above-described interval into consideration, the surface size of the intermediary image bearing member  15 A must be made slightly larger than the size corresponding to two pages of the A4-sized recording medium. The length of the above-described interval is determined based upon a time in which a recording medium P fed from the sheet feeding device  23  is once stopped by the registration roller pair  26  and is conveyed again having a timing that the recording medium P is registered with an image to be transferred thereupon. 
   Further, as described earlier, a one-side printing mode for transferring a visible image transferred onto an intermediary transfer member from an image bearing member onto one side of a recording medium without using an intermediary image bearing member can be selected. In this mode, even when the circumferential length of an intermediary image bearing member is short, a color image longer than the circumferential length of the intermediary image bearing member can be formed on one side of a recording medium. Accordingly, when using the intermediary image bearing member  15  that is short in circumferential length, a color image large in size can be formed on a recording medium by using the above-described one-side printing mode. 
   Furthermore, when the image forming apparatus of the present invention is connected with a host computer as described later, by configuring the control unit E 2  of the image forming apparatus such that an image forming operation is not performed when an instruction for forming an image on each side of a recording medium that is larger in size than the surface size of an intermediary image bearing member of the image forming apparatus is given by the host computer or by the operation panel OP provided to the main body housing  1  of the image forming apparatus, only those images that can be correctly accommodated in the recording medium can be formed at any time. 
     FIG. 10  also illustrates an example of an image forming system configured by connecting the image forming apparatuses  100 ,  100 A and  100 B of the present invention with a host computer HC via a network. The image forming apparatuses  100 ,  100 A and  100 B mainly perform a function of an outputting device (a printer) for the host computer HC. Each of the image forming apparatuses  100 ,  100 A and  100 B and the host computer HC may be connected with each other via wireless communication instead of a cable. Process conditions, etc. for forming an outputting image with one of the image forming apparatuses  100 ,  100 A and  100 B are inputted in accordance with guidance messages displayed in a display of the host computer HC. Statuses of the image forming apparatuses  100 ,  100 A and  100 B can be also displayed in the display of the host computer HC. Control instructions can be inputted through the operation panels OP of the image forming apparatuses  100 ,  100 A and  100 B or a keyboard of the host computer HC. For example, when performing both-side printing in which an image is formed on each side of a recording medium, an instruction for the both-side printing may be inputted by operating a both-side printing button provided on each operation panel OP or by operating the keyboard of the host computer HC. Further, when a plurality of the sheet feeding cassettes  24  accommodating various types of papers are provided in the image forming apparatus  100 ,  100 A or  100 B, selection of a special paper among the various types of paper accommodated in the sheet feeding cassettes  24  or selection of one of the sheet feeding cassettes  24  maybe also similarly performed by operating a sheet selecting button of the operation panel OP or by operating the keyboard of the host computer HC. 
   As described above, in the image forming system configured by connecting the image forming apparatus of the present invention with the host computer HC, information can be printed on a recording medium according to the object of printing and the kind of the recording medium. Further, image forming conditions in the image forming apparatus can be set at the image forming apparatus or from a location remote from the image forming apparatus by the host computer HC. Further, when the recognizing device for automatically recognizing which of the belt units  64 ,  64 A and  64 B is installed in the installing part IP of the main body housing  1  is provided, as described above, the recognition result can be provided to the host computer HC and the size of an intermediary image bearing member included in the installed one of the belt units  64 ,  64 A and  64 B can be recognized from the location of host computer HC that is remote from the image forming apparatus. 
   Each of the image forming apparatuses of  FIG. 1  and  FIG. 6  includes the plurality of image bearing members  2 Y,  2 M,  2 C and  2 BK. However, as illustrated in  FIG. 19 , the image forming apparatus may be configured to include a single image bearing member  2  contacting the intermediary transfer member  3 . Visible toner images of different colors are sequentially formed on the image bearing member  2  by an image forming device  6 , which are then transferred onto the intermediary transfer member  3  being superimposed one upon another. That is, after discharging a surface of the image bearing member  2  rotating in the arrow direction by the discharging device  7 , the surface of the image bearing member  2  is charged by the charging device  8 , the charged surface of the image bearing member  2  is selectively irradiated by a light emitted by the exposure device  9 , and thereby an electrostatic latent image is formed on the image bearing member  2 . The electrostatic latent image is made visible as a yellow toner image by a first developing device  11 Y of the rotatably supported developing device  11 . The yellow toner image is transferred by a function of the transfer roller  12  onto the intermediary transfer member  3  rotating in the direction indicated by the arrow A. A surface of the image bearing member  2  after transfer of the yellow toner image is cleaned by the cleaning device  14 . 
   In substantially the same manner, a magenta toner image, a cyan toner image, and a black toner image are sequentially formed on the image bearing member  2  by a magenta developing device  11 M, a cyan developing device  11 C, and a black developing device  11 BK of the developing device  11 , and these visible toner images are sequentially transferred onto the intermediary transfer member  3  onto which the yellow toner image has been transferred, being superimposed one upon another and thereby forming the first image on the intermediary transfer member  3 . Residual toner adhering to a surface of the intermediary transfer member  3  after transfer of the first image onto the intermediary image bearing member  15  is removed by the cleaning device  22 . In the cleaning device  22  illustrated in  FIG. 19 , a brush roller  59  provided as a cleaning member of the cleaning device  22  is supported to contact and separate from the intermediary transfer member  3 . When a visible image on the intermediary transfer member  3  passes the cleaning device  22 , the brush roller  59  of the cleaning device  22  is separated from a surface of the intermediary transfer member  3 . The brush roller  59  contacts a surface of the intermediary transfer member  3  only when removing residual toner on the intermediary transfer member  3 . 
   The other parts of the configuration and operation of the image forming apparatus are substantially the same as those of the image forming apparatus of  FIG. 1 . The above-described first image is transferred onto the intermediary image bearing member  15 , the first image is transferred onto one side of a recording medium P, the second image formed on the intermediary transfer member  3  by superimposed visible images is transferred onto the other side of the recording medium P, and the first and second images are fixed to the recording medium P by the fixing device  28 . The intermediary image bearing member  15  is supported so as to contact and separate from the intermediary transfer member  3 , and when the first image on the intermediary transfer member  3  passes the intermediary image bearing member  15 , the intermediary image bearing member  15  separates from the intermediary transfer member  3 , and when transferring the first image on the intermediary transfer member  3  onto the intermediary image bearing member  15  and when transferring the first image and the second image onto one and the other sides of the recording medium P, the intermediary image bearing member  15  contacts the intermediary transfer member  3  directly or via the recording medium P. In  FIG. 19 , each component of the image forming apparatus is denoted by the same reference signs used for a corresponding component of the image forming apparatus illustrated in  FIG. 1 , and further description of the apparatus is omitted. 
   Also, in the image forming apparatus of  FIG. 19 , the above-described configurations of the present invention can be adopted, so that the belt unit  64  including the intermediary image bearing member  15  can be exchanged with another belt unit including another intermediary image bearing member that is different in size from the intermediary image bearing member  15 . Thereby, an image forming apparatus meeting specific a need of the user can be easily configured. 
   In each of the image forming apparatuses illustrated in  FIG. 1  and  FIG. 19 , the image bearing members  2 Y,  2 M,  2 C and  2 BK and the image bearing member  2 , and the image forming devices  6 Y,  6 M,  6 C and  6 BK and the image forming device  6  are arranged above the intermediary transfer member  3 , respectively, however, these components can be arranged below the intermediary transfer member  3 , as in an image forming apparatus described below with reference to  FIG. 20 . When the image bearing members  2 Y,  2 M,  2 C and  2 BK and the image bearing member  2 , and the image forming devices  6 Y,  6 M,  6 C and  6 BK and the image forming device  6  are arranged below the intermediary transfer member  3 , respectively, the time for an image on the intermediary transfer member  3  to reach a transfer position where the image is transferred onto the intermediary image bearing member  15  or onto a recording medium can be decreased, so that image forming efficiency can be enhanced. 
     FIG. 20  is a vertical cross section illustrating such an image forming apparatus configured as a printer, and  FIG. 22  is a vertical cross section of still another image forming apparatus configured as a printer that is configured differently from the one of  FIG. 20 . 
   The image forming apparatus illustrated in  FIG. 1  and  FIG. 6  and the image forming apparatus illustrated in  FIG. 20  are similar to each other in basic structure. Accordingly, only those portions of the image forming apparatus of  FIG. 20 , different from corresponding portions of the image forming apparatus of  FIG. 1  and  FIG. 6 , will be described. Respective components of the image forming apparatus of  FIG. 20  are denoted by same reference signs as those used for corresponding components of the image forming apparatus illustrated in  FIG. 1  and  FIG. 6 . 
   In the image forming apparatus illustrated in  FIG. 20 , at least one image bearing member driven to rotate in the clockwise direction, in the illustrated example, the first through fourth image bearing members  2 Y,  2 M,  2 C and  2 BK are arranged inside of the main body housing  1 , as in the image forming apparatus of  FIG. 1 , and visible toner images of different colors, i.e., yellow, magenta, cyan and black colors, are formed by the image forming devices  6 Y,  6 M,  6 C and  6 BK on the image bearing members  2 Y,  2 M,  2 C and  2 BK, which are then transferred onto the intermediary transfer member  3  formed as an endless belt spanned around the supporting rollers  4  and  5  and rotated in the direction of the arrow A, so as to be superimposed one upon another, thereby forming the first image thereupon. Thus, the image forming apparatus in this example also includes at least one image bearing member on which a visible image is formed, a developing device to form the visible image on the at least one image bearing member, and an intermediary transfer member onto which visible images of different colors formed on the at least one image bearing member are transferred being superimposed one upon another, and the configurations of the image bearing member and the intermediary transfer member, such as material thereof, are substantially the same as those of the image forming apparatus described above. A laser writing device emitting a laser light L is used for the exposure device  9  illustrated in  FIG. 20 , however, an exposure device including an LED array may be used as in the image forming apparatus illustrated in  FIG. 1  and  FIG. 6 . 
   The intermediary image bearing member  15  formed as an endless belt is arranged at the right side of the intermediary transfer member  3  in  FIG. 20 , and the intermediary image bearing member  15  is spanned around the supporting rollers  16 ,  17  and  18  and is driven to rotate in the direction of the arrow B in synchronism with the intermediary transfer member  3 . Material and characteristics of the intermediary image bearing member  15  are substantially the same as those of the intermediary image bearing member  15  illustrated in  FIG. 1  and  FIG. 6 . The transfer roller  20  acting as a transfer device and the backing roller  21  are arranged in the positions opposing the supporting roller  5  for the intermediary transfer member  3  inside of the intermediary image bearing member  15  to contact a rear side of the intermediary image bearing member  15 . Thereby, the intermediary transfer member  3  and the intermediary image bearing member  15  contact each other with an appropriate nip formed between them. 
   A transfer voltage is applied to the transfer roller  20 , so that the first image on the intermediary transfer member  3  is transferred onto the intermediary image bearing member  15 , and residual toner adhering on a surface of the intermediary transfer member  3  after transfer of the first image is removed by a cleaning member  22 A of the cleaning device  22 . 
   Subsequently, toner images of different colors formed on the image bearing members  2 Y,  2 M,  2 C and  2 BK are transferred onto the intermediary transfer member  3 , being superimposed one upon another, so that the second image is formed on the intermediary transfer member  3 . 
   The sheet feeding device  23  illustrated in  FIG. 20  includes two sheet feeding cassettes  24  and  24 A, each stacking and accommodating therein a recording medium P, e.g., a transfer sheet, a resin sheet, etc., and two feeding roller  25  and  25 A to feed out the recording medium P from the sheet feeding cassettes  24  and  24 A. Either of the two feeding rollers  25  and  25 A rotates while contacting a surface of a top recording medium P, thereby the top recording medium P being fed out to the registration roller pair  26 . The registration roller pair  26  feeds out the recording medium P upward with a predetermined timing, and by a function of the corona discharging device  27  serving as a transfer device arranged opposing the intermediary image bearing member  15 , the first image on the intermediary image bearing member  15  is transferred onto one side of the recording medium P, and by a function of the transfer roller  20 , the second image on the intermediary transfer member  3  is transferred onto the other side of the recording medium P. A surface of the intermediary transfer member  3 , after transferring the second image, is cleaned by the cleaning device  22 . Residual toner adhering to a surface of the intermediary image bearing member  15  is removed by the cleaning blade  54  of the cleaning device  32 . The cleaning blade  54  also separates from a surface of the intermediary image bearing member  15  when the first image on the intermediary image bearing member  15  passes the cleaning blade  54 . 
   The recording medium P on which the first and the second images have been transferred passes the fixing device  28 , and at that time, both of the first and second images are fixed onto respective surfaces of the recording medium P. The recording medium P is then fed by the sheet feeding roller pair  53  onto the sheet expelling part  31  as indicated by the arrow C. 
   The image forming apparatus illustrated in  FIG. 20  also includes the toner containers  36 Y,  36 M,  36 C and  36 BK containing yellow toner, magenta toner, cyan toner and black toner for replenishment, respectively. These containers are provided in the toner container accommodating space S above the intermediary transfer member  3 . A cover  31 A constituting a part of the sheet expelling part  31  is opened upward in figure (as in an image forming apparatus illustrated at a lower part of  FIG. 24 ), so that each of the toner containers  36 Y,  36 M,  36 C and  36 BK can be replaced with a new one. The cover  31 A is supported at one end side thereof by a rotating fulcrum  137  so as to open around the fulcrum  137 . Because this rotating fulcrum  137  is located at the side of the sheet feeding roller pair  53 , even if the cover  31 A is opened when a recording medium P on which an image has been recorded exists on the sheet expelling part  31 , such a trouble that the recording medium P falls down or the page sequence thereof goes out of order will never occur. In  FIG. 20 , numeral  34  denotes a knob of the cover  31 A, and the cover  31 A can be opened by grabbing the knob  34  by hand. The knob  34  is arranged in such a position that the knob  34  will not hinder conveyance and stacking of a recording medium P being fed onto the sheet expelling part  31 . 
   The image forming apparatus illustrated at a lower part of  FIG. 24  is in a condition where a front door DA of the main body housing  1  is opened so that maintenance work for the inside part of the image forming apparatus can be performed, and further the sheet feeding cassette  24 A at the lower side is drawn out so that a recording medium P can be replenished or replaced with another one. By opening the front cover DA, image bearing members, an intermediary transfer member, and other components around them can be drawn out forward while being guided by a guide rail (not shown), with an exposure device being left inside the main body housing  1 , so that the image bearing members and the intermediary transfer member can be removed from the main body housing  1  in this condition. The front door DA is supported by a part of the main body housing  1  via a hinge arranged in a vertical direction, so that even when the front door DA is opened, visibility of other parts of the image forming apparatus below the front door DA is never blocked. 
   Further, as illustrated in  FIG. 20 , a heat insulating member W is inserted between the toner container accommodating space Sand the fixing device  28 . Thereby, the toner for replenishment can be prevented from being melted and fixed as a result of repetition of heating and cooling. Resin, hair transplanted resin, or a member in which a plurality of layers of resin or hair transplanted resin are formed such that an air layer is formed therein may be used for the heat insulating member W. Provision of an air circulating path for circulating air to pass a fan F 2  can also be effective for preventing the replenishing toner from being melted and fixed. 
   The above-described configurations can be adopted in the image forming apparatus illustrated in  FIG. 1  and  FIG. 6 . 
   In the image forming apparatus illustrated in  FIG. 20 , at least the intermediary image bearing member  15  and the supporting rollers  15 ,  16  and  17  around which the intermediary image bearing member  15  is spanned around are integrated with each other to be formed as the belt unit  64 . In the example illustrated in  FIG. 20 , the transfer roller  20  and the corona discharging device  27  serving as the transfer devices, the backing roller  21 , and the cleaning device  32  also constitute components of the belt unit  64 , and these components  15 ,  16 ,  17 ,  18 ,  20 ,  21  and  27  are integrally attached to a case  150  to be configured as the belt unit  64 . 
   In contrast, in the image forming apparatus illustrated in  FIG. 22 , instead of the belt unit  64  illustrated in  FIG. 20 , a transfer device  10  is provided. The transfer device  10  may be configured only by a transfer roller or a transfer brush to which a transfer voltage is applied or a corona discharging device. However, the transfer device  10  illustrated in  FIG. 22  includes a plurality of supporting rollers  116 ,  117  and  118 , an endless transfer belt  115  which is spanned around the supporting rollers  116 ,  117  and  118  to be driven to rotate in the direction of the arrow B, a transfer roller  120  functioning as a transfer device to transfer a visible image on the intermediary transfer member  3  onto a recording medium, and a cleaning device  132  cleaning a surface of the transfer belt  115 . In this case also, the supporting rollers  116 ,  117  and  118 , and the endless transfer belt  115  spanned around the supporting rollers  116 ,  117  and  118  to be driven to rotate can be integrated with each other to be formed as a transfer unit  164 . In the example illustrated in  FIG. 22 , the supporting rollers  116 ,  117  and  118 , the transfer belt  115 , the transfer roller  120 , and the cleaning device  132  are integrally attached to a case  250  to be configured as the transfer unit  164 . In the image forming apparatus illustrated in  FIG. 22 , the corona discharging device  27  illustrated in  FIG. 20  is not provided. 
   The transfer roller  120  contacts a backside of the transfer belt  115  and is positioned to substantially oppose the supporting roller  5  around which the intermediary transfer member  3  is spanned. In this example, the transfer roller  120  is used as a transfer device, however, any other appropriate transfer devices, such as a corona discharging device, etc. can be also used. However, by using the transfer roller  120 , because the transfer belt  115  can be pressed against a part of the intermediary transfer member  3  spanned around the supporting roller  5 , transfer efficiency can be enhanced. Further, the cleaning device  132  includes a cleaning blade  154  as an example of a cleaning member, and the cleaning blade  154  is pressed to contact a surface of the transfer belt  115  to clean the surface of the transfer belt  115 . The other portions of the image forming apparatus illustrated in  FIG. 22  are substantially the same as those of the image forming apparatus illustrated in  FIG. 20 , and therefore, respective components of the image forming apparatus illustrated in  FIG. 22  are denoted by the same references signs as those used for corresponding components illustrated in  FIG. 20 , and description thereof is omitted. 
   Also, in the image forming apparatus illustrated in  FIG. 22 , substantially in the same manner as in the image forming apparatus of  FIG. 20 , visible toner images of different colors, i.e., yellow, magenta, cyan and black colors, are formed by the image forming devices  6 Y,  6 M,  6 C and  6 BK on the image bearing members  2 Y,  2 M,  2 C and  2 BK, which are then transferred onto the intermediary transfer member  3  formed as an endless belt spanned around the supporting rollers  4  and  5  and rotated in the direction of the arrow A, so as to be superimposed one upon another, thereby forming the first image thereupon. The surfaces of the image bearing members  2 Y,  2 M,  2 C and  2 BK after transfer of the toner images are cleaned by the cleaning devices  14 . Thus, a superimposed visible image is formed on the intermediary transfer member  3 . 
   A recording medium P fed out from the sheet feeding device  23  and conveyed by the registration roller pair  26  in a predetermined timing passes a nip part, where the intermediary transfer member  3  and the transfer belt  115 , rotating in synchronism with each other in the directions of arrows, respectively, contact each other, to be further conveyed upward. At this time, a transfer voltage having a polarity opposite to that of the toner of the superimposed visible image on the intermediary transfer member  3  (in the illustrated example, a plus polarity) is applied to the transfer roller  120 , and thereby the superimposed visible image on the intermediary transfer member  3  is transferred onto one side of the recording medium being conveyed. Toner adhering to a surface of the transfer belt  115  is removed by the cleaning blade  154  of the cleaning device  132 , so that the surface of the transfer belt  115  is cleaned. Thus, the transfer device  10  in this example is configured to cause a recording medium P to pass the nip part of the transfer belt  115  and the intermediary transfer member  3  so that a visible image on the intermediary transfer member  3  is transferred onto one side of the recording medium P. 
   The transfer belt  115  is configured as a belt having conductivity enabling transfer of a visible toner image from the intermediary transfer member  3  onto a recording medium P. By using the transfer belt  115 , a relatively large nip part can be formed between the transfer belt  115  and the intermediary transfer member  3 , so that the visible image is efficiently transferred from the intermediary transfer member  3  onto the recording medium P at the nip part, and further the recording medium P is securely conveyed upward continuously thereafter. 
   The recording medium P onto which the visible image has been transferred is conveyed upward while being in close contact with the transfer belt  115 , and after having been separated from the transfer belt  115 , the recording medium P passes the fixing device  28 , where the visible image transferred onto the one side of the recording medium P is fixed thereon. The recording medium P passing the fixing device  28  is fed by the sheet feeding roller pair  53  onto the sheet expelling part  31  as indicated by the arrow C. At this time, the recording medium P is fed onto the sheet expelling part  31  with the side thereof carrying the fixed visible image face down. Residual toner adhering to a surface of the intermediary transfer member  3  after transferring the visible image onto the recording medium P is removed by the cleaning member  22 A of the cleaning device  22 , so that the surface of the intermediary transfer member  3  is cleaned. 
   The intermediary image bearing member  15  illustrated in  FIG. 20  must have a circumferential length for one page of the first image. In contrast, the transfer belt  115  illustrated in  FIG. 22  does not bear an image thereupon and therefore does not need to have a circumferential length for one page of the first image. Thus, the transfer belt  115  and the intermediary image bearing member  15  differ from each other in configuration because of the difference in function. 
   According to the image forming apparatus of  FIG. 20 , a color image can be formed on each side of a recording medium. On the other hand, the image forming apparatus of  FIG. 22  can form a color image only on one side of a recording medium. However, the image forming apparatus of  FIG. 20  is more complicated in configuration and thereby is more expensive than the image forming apparatus of  FIG. 22 . Thus, the image forming apparatuses of  FIG. 20  and  FIG. 22  have merits and demerits respectively, and a user obtains either of the image forming apparatuses meeting its needs taking into consideration such merits and demerits. In this case, however, when a user who has obtained the image forming apparatus of  FIG. 22  must later also obtain the image forming apparatus of  FIG. 20 , the financial burden on such a user is increased. 
   Accordingly, each of the image forming apparatuses of  FIG. 20  and  FIG. 22  includes the above-described installing mechanism configured, in this case, to install either of the transfer device  10  illustrated in  FIG. 20  and the intermediary image bearing member  15  illustrated in  FIG. 22  in the installing part IP of the main body housing  1  in a detachable manner. Thereby, the transfer device  10  and the intermediary image bearing member  15  can be freely exchanged with each other in the image forming apparatuses of  FIG. 20a  and  FIG. 22 . In the description below, the belt unit  64  including the intermediary image bearing member  15  is called the first belt unit and the transfer unit  164  including the transfer belt  115  is called the second belt unit as necessary. 
   In each of the image forming apparatuses illustrated in  FIG. 20  and  FIG. 22 , as in the image forming apparatus illustrated in  FIG. 1  and  FIG. 6 , the main body housing  1  includes the fixed housing  1 A and the movable housing  1 B supported by the fixed housing  1 A to open and close relative to the fixed housing  1 A, and further, the receiving and guiding mechanism is provided to the movable housing  1 B, so that when the movable housing  1 B is in the opened position, the transfer device  10  or the intermediary image bearing member  15  can be received by and taken out from the movable housing  1 B. More specifically, the movable housing  1 B is supported via the supporting axis  35  by the fixed housing  1 A so as to rotate to open and close, and the first belt unit  64  illustrated in  FIG. 20  and the second belt unit  164  illustrated in  FIG. 22  are received by the movable housing  1 B in a freely detachable manner, respectively.  FIG. 21  and  FIG. 23  illustrate states that the movable housing  1 B illustrated in  FIG. 20  and the movable housing  1 B illustrated in  FIG. 22  are rotated to the opened positions, respectively. At this time, because the space above the movable housing  1 B is opened, the first and second belt units  64  and  164  can be lifted upward to be taken out from the movable cover  1 B, respectively. 
   Because the receiving and guiding mechanism provided to the movable housing  1 B is configured such that either of the first belt unit  64  including the intermediary image bearing member  15  and the second belt unit  164  including the transfer belt  115  can be received, when a user using the image forming apparatus of  FIG. 22  must obtain an image forming apparatus that can form a color image on each side of a recording medium, the user can obtain the image forming apparatus that can form a color image on each side of a recording medium by obtaining the first belt unit  64  including the intermediary image bearing member  15 . That is, the user opens the removable housing  1 B, thereby removing the second belt unit  164  including the intermediary image bearing member  115  from the installing part IP of the main body housing  1 , as illustrated in  FIG. 23 , and takes out the second belt unit  164  including the transfer belt  115 . The user then causes the newly obtained first belt unit  64  including the intermediary image bearing member  15  to be received by the movable housing  1 B, and closes the movable housing  1 B, and thereby the first belt unit  64  including the intermediary image bearing member  15  is installed in the installing IP of the main body housing  1  of the image forming apparatus illustrated in  FIG. 22 , so that the image forming apparatus configured as illustrated in  FIG. 20  is realized. The first belt unit  64  including the intermediary image bearing member  15  may be sometimes exchanged to obtain the second belt unit  164  including the transfer belt  115  kept, or sold separately, as the case may be. Thus, by exchanging a part of an image forming apparatus with another part, the image forming apparatus can be freely configured to form an image on only on one side of a recording medium or to form an image one each side of a recording medium. 
   Further, when manufacturing image forming apparatuses, by installing either the first belt unit  64  and the second belt unit  164  in the main body housing  1  of each image forming apparatus, image forming apparatuses having different functions can be manufactured. Thereby, meeting the diverse needs of users can be easily realized. 
   Futhermore, by opening the movable housing  1 B, a recording medium conveying path is opened to a large extent, so that a recording medium jammed in the conveying path can easily be cleared. Thus, the movable housing  1 B can be opened for other purposes than exchanging of the first belt unit  64  and the second belt unit  164  with each other, thereby facilitating performance of necessary works such as paper jam removal. 
   The image forming apparatuses of  FIG. 20  and  FIG. 22  differ from each other as described above in the operations of forming visible images, the timings of conveying a recording medium P from the sheet feeding device  23 , the operations of the cleaning devices  23  and  132 , etc. Further, the fixing temperature when the fixing device  28  illustrated in  FIG. 22  fixes a visible image onto a recording medium P is different from that when the fixing device  28  of  FIG. 20  fixes a visible image onto a recording medium P. The fixing temperature is set to 170° C. by the former fixing device  28  and to 180° C. by the latter fixing device  28 . In the image forming apparatus of  FIG. 20 , because visible images on both sides of the recording medium P are fixed onto respective sides of the recording medium P, the fixing temperature is set relatively high so that inferior fixing is avoided, and in the image forming apparatus of  FIG. 22 , because a visible image on one side of the recording medium P is fixed onto the recording medium P, the fixing temperature is set relatively low. That is, the fixing temperature of the fixing device  28  for fixing a visible image onto a recording medium P is set higher when the intermediary image bearing member  15  is installed than when the transfer device  10  is installed in the main body housing  1 . When forming a color image on one side of a recording medium P with the image forming apparatus of  FIG. 20 , the fixing temperature may be set to the same as that of the image forming apparatus of  FIG. 22 . 
   As described above, because the operations of the image forming apparatuses of  FIG. 20  and  FIG. 22  differ from each other, each of the image forming apparatuses must be controlled, when the transfer device  10  (the second belt unit  164 ) and the intermediary image bearing member  15  (the first belt unit  64 ) are exchanged with each other, to operate in a manner corresponding to the first belt unit  64  or the second belt unit  164 , installed in the installing part IP of the main body housing  1 . 
   Accordingly, a recognition device such as a photo-sensor and a micro-switch (not shown) for recognizing if the transfer device  10  is installed or if the intermediary image bearing member  15  is installed in the installing part IP of the main body housing  1  is provided in the main body housing  1 , so that according to a result of recognition with the recognition device, control when the transfer device  10  is installed or control when the intermediary image bearing member  15  is installed in the main body housing  1  is performed. Alternatively, as illustrated in  FIG. 24 , an insertion inlet  155  for inserting an IC card  154 A as an example of a storage medium may be provided to the main body housing  1 . When the intermediary image bearing member  15  is installed in the main body housing  1  in place of the transfer device  10 , by inserting the IC card  154 A into the insertion inlet  155 , the control when the intermediary image bearing member  15  is installed in the main body housing  1  is performed. Thus, by adopting either of the above-described configurations in an image forming apparatus, the image forming apparatus can be controlled to operate in a corresponding manner after the transfer device  10  or the intermediary image bearing member  115  has been exchanged with the intermediary image bearing member  15  or the transfer device  10 . 
   Further, the operation panel OP ( FIG. 24 ) provided to an outer surface of the main body housing  1  may display information informing the user that the intermediary image bearing member  15  is installed in place of the transfer device  10  in the main body housing  1 . Thereby, a user can be informed that an image can be formed on each side of a recording medium, so that the user will be never confused. 
     FIG. 24  also illustrates an image forming system configured by connecting the image forming apparatus of the present invention illustrated in  FIG. 20  and  FIG. 22  with the host computer HC via a network as in the image forming system described above referring to  FIG. 10 . In this case, when the intermediary image bearing member  15  is installed in place of the transfer device  10  in the main body housing  1  of the image forming apparatus, the information informing that the intermediary image bearing member  15  is installed in place of the transfer device  10  in the image forming apparatus can be displayed on the operation panel OP of the image forming apparatus and/or the display of the host computer HC. 
   The image forming apparatuses illustrated in  FIG. 20  and  FIG. 22  include the plurality of image bearing members  2 Y,  2 M,  2 C and  2 BK, respectively. However, as illustrated in  FIG. 25  and  FIG. 26 , the image forming apparatuses may be configured to include the single image bearing member  2  contacting the intermediary transfer member  3  as in the image forming apparatus described with reference to  FIG. 19 . Visible toner images of different colors are sequentially formed on the single image bearing member  2  by the image forming device  6 , which are then transferred onto the intermediary transfer member  3  superimposing one upon another, as described above with reference to  FIG. 19 . 
   The image forming apparatus of  FIG. 25  includes the transfer device  10  as in the image forming apparatus of  FIG. 22 , and the image forming apparatus of  FIG. 27  includes the intermediary image bearing member  15  as in the image forming apparatus of  FIG. 20 . The transfer device  10  and the intermediary image bearing member  15  of the image forming apparatuses illustrated in  FIG. 25  and  FIG. 26  are supported so as to freely contact and separate from the intermediary transfer member  3 , respectively. The transfer belt  115  of the transfer device  10  and the intermediary image bearing member  15  are separated from the intermediary transfer member  3  except when transferring a visible image onto a recording medium P or the intermediary image bearing member  15  as described below. 
   In the image forming apparatus of  FIG. 25 , a superimposed visible image on the intermediary transfer member  3  is transferred by a function of the transfer roller  120  only onto one side of a recording medium P conveyed from the sheet feeding device  23 . On the other hand, in the image forming apparatus of  FIG. 26 , the first image formed on the intermediary tranfer member  3  by superimposed visible images is once transferred onto the intermediary image bearing member  15  rotating in the directon indicated by the arrow B by a function of the transfer roller  20 . The first image is transferred by a function of the corona discharging device  27  onto the side of the recording medium P, and subsequently the second image formed by superimposed visible images on the intermediary transfer member  3  is transferred by a function of the transfer roller  20  onto the other side of the recording medium P. Both images on the recording medium P are fixed by the fixing device  28 , and the recording medium P after fixing the images thereon is fed onto the sheet expelling part  31 . Residual toner adhering to a surface of the intermediary transfer member  3  after transfer of the first image is removed by the cleaning device  22 . A cleaning member  22 A of the cleaning device  22  illustrated in  FIG. 25  and  FIG. 26  is supported to contact and separate from the intermediary transfer member  3 . When a visible image on the intermediary transfer member  22  is separated from a surface of the intermediary transfer member  3 . The cleaning member  22 A contacts a surface of the intermediary transfer member  3  only when removing residual toner on the intermediary transfer member  3 . 
   In the image forming apparatuses of  FIG. 25  and  FIG. 26  also, the second belt unit  164  including the transfer device  10  and the first belt unit  64  including the intermediary image bearing member  15  can be exchanged with each other. Thereby, an image forming apparatus meeting the needs of the user can be configured. Further, by connecting a host computer with each of the image forming apparatuses of  FIG. 25  and  FIG. 26 , an image forming system similar to the one described above can be realized. 
   The other parts of the configuration and operation of the image forming apparatuses of  FIG. 25  and  FIG. 26  are substantially the same as those of the image forming apparatuses of  FIG. 20  and  FIG. 22 , and respective components of the image forming apparatuses of  FIG. 25  and  FIG. 26  are denoted by same reference signs used for corresponding components in  FIG. 20  and  FIG. 22 . Therefore, further description is omitted. 
   As described above, each of the image forming apparatuses illustrated in  FIG. 20  through  FIG. 26  includes at least one of the image bearing member  2 Y,  2 M,  2 C and  2 BK or the single image bearing member  2 , on which visible images are formed, the image forming devices  6 Y,  6 M,  6 C and  6 BK or the image forming device  6  configured to form visible images on the at least one of the image bearing members  2 Y,  2 M,  2 C and  2 BK or the single image bearing member  2 , and the intermediary transfer member  3  onto which visible images of different colors formed on the at least one of the image bearing members  2 Y,  2 M,  2 C and  2 BK or the single image bearing member  2  are transferred superimposing one upon another, forming the first image thereupon. Further, the image forming apparatus includes the installing part IP provided in the main body housing  1  and configured such that either of the transfer device  10  configured to transfer, when installed in the installing part IP of the main body housing  1 , a superimposed visible image on the intermediary transfer member  3  onto a recording medium P and the intermediary image bearing member  15  configured such that, when installed in the installing part IP, the first image formed on the intermediary transfer member  3  is transferred thereupon, can be installed therein in a detachable manner. When the intermediary image bearing member  15  is installed in the installing part IP of the main body housing  1 , the image forming apparatus is configured such that the first image on the intermediary image bearing member  15  is transferred onto one side of the recording medium P and a second image formed on the intermediary transfer member  3  by superimposed visible images subsequently transferred from the at least one of the image bearing members  2 Y,  2 M,  2 C and  2 BK or the single image bearing member  2  is transferred onto the other side of the recording medium P. Thus, the image forming apparatus can be easily configured to be another image forming apparatus meeting a need of the user. 
   Further, as illustrated in  FIG. 27  and  FIG. 28 , the image forming apparatuses of the present invention can be configured omitting an intermediary transfer member. In an image forming apparatus illustrated in  FIG. 27 , the transfer belt  115  of the transfer device  10  is arranged to oppose the plurality of image bearing members  2 Y,  2 M,  2 C and  2 BK, and the transfer belt  115  is rotated in the direction of the arrow B. Yellow toner images, magenta toner images, cyan toner images, and black toner images are formed by the image forming devices  6 Y,  6 M,  6 C and  6 BK on the image bearing members  2 Y,  2 M,  2 C and  2 BK, respectively, as in the image forming apparatuses of  FIG. 20  and  FIG. 22 . These toner images of different colors are transferred onto one side of a recording medium P conveyed from the sheet feeding device  23  by a function of each transfer roller  120  of the transfer device  10 . The recording medium P is caused to pass respective nip parts between the transfer belt  115  and the image bearing members  2 Y,  2 M,  2 C and  2 BK, so that the visible toner images on the image bearing members  2 Y,  2 M,  2 C and  2 BK are transferred onto the recording medium P to be superimposed one upon another thereon. 
   In the image forming apparatus illustrated in  FIG. 28 , the intermediary image bearing member  15  is arranged to oppose the image bearing members  2 Y,  2 M,  2 C and  2 BK, and visible toner images of different colors formed on the image bearing members  2 Y,  2 M,  2 C and  2 BK, respectively, are transferred onto the intermediary image bearing member  15  being rotated in the direction of the arrow B to be superimposed one upon another by a function of the transfer rollers  20 , thereby forming the first image thereupon. The first image is then transferred by a function of the corona discharging device  27  onto one side of a recording medium P conveyed from the sheet feeding device  23 , and visible toner images of different colors subsequently formed on the image bearing members  2 Y,  2 M,  2 C and  2 BK are transferred onto the other side of the recording medium P by a function of the transfer rollers  20  to be superimposed one upon another, thereby forming the second image thereupon. Both images on the recording medium P are fixed by the fixing device  28  onto the recording medium P, and the recording medium P is fed onto the sheet expelling part  31 . The image forming apparatus of  FIG. 28  can also form a color image only on one side of a recording medium P. 
   As described above, the image forming apparatuses of  FIG. 27  and  FIG. 28  differ from the image forming apparatuses of  FIG. 20  through  FIG. 24  in that visible images formed on the image bearing members  2 Y,  2 M,  2 C, and  2 BK are transferred onto a recording medium P without use of an intermediary transfer member, and other parts of the image forming apparatuses of  FIG. 27  and  FIG. 28  are substantially the same as those of the image forming apparatuses of  FIG. 20  through  FIG. 24 . Further, as in the image forming apparatuses of  FIG. 20  and  FIG. 22 , in the image forming apparatus of  FIG. 27 , the second belt unit  164  is configured by the transfer device  10 , etc., and in the image forming apparatus of  FIG. 28 , the first belt unit  64  is configured by the intermediary image bearing member  15 , etc., and the installing mechanism configured to install in the main body housing  1  either of the second belt unit  164  including the transfer device  10  and the first belt unit  64  including the intermediary transfer member  15  is provided in the image forming apparatuses of  FIG. 27  and  FIG. 28 , respectively. Thereby, the second belt unit  164  including the transfer device  10  and the first belt unit  64  including the intermediary image bearing member  15  can be exchanged with each other in the image forming apparatuses of  FIG. 27  and  FIG. 28 . Thus, an image forming apparatus meeting a need of a user can be easily configured. Respective components of the image forming apparatuses of  FIG. 28  and  FIG. 27  are denoted by the same reference signs as those used for corresponding components of the image forming apparatuses of  FIG. 20  and  FIG. 22 . 
   Further, in the image forming apparatuses of  FIG. 27  and  FIG. 28 , the plurality of image bearing members  2 Y,  2 M,  2 C and  2 BK are provided. However, each of the apparatuses may be configured to include a single image bearing member and toner images formed on the single image bearing member may be sequentially transferred onto a recording medium, as in the image forming apparatuses described above. 
   As described above, each of the image forming apparatuses described above referring to  FIG. 27  and  FIG. 28  includes at least one of the image bearing members  2 Y,  2 M,  2 C and  2 BK on which visible images are formed, the image forming devices  6 Y,  6 M,  6 C and  6 BK configured to form visible images on the at least one of the image bearing members  2 Y,  2 M,  2 C and  2 BK, and the installing part IP provided in the main body housing  1  and configured such that either of the transfer device  10  configured to transfer a visible images formed on the at least one of the image bearing members  2 Y,  2 M,  2 C and  2 BK onto a recording medium P and the intermediary image bearing member  15  on which visible images formed on the at least one of the image bearing members  2 Y,  2 M,  2 C and  2 BK are transferred to be superimposed one upon another, thereby forming a first image thereupon, can be installed therein. The image forming apparatus is configured such that when the intermediary image bearing member  15  is installed in the installing part IP of the main body housing  1 , the first image formed on the intermediary image bearing member  15  is transferred onto one side of the recording medium P and a second image is formed on the other side of the recording medium P by visible images subsequently transferred from the at least one of the image bearing members  2 Y,  2 M,  2 C and  2 BK to be superimposed one upon another. 
   In the image forming apparatuses illustrated in  FIG. 20  through  FIG. 28 , as described above, the installing mechanism is configured such that either of the transfer device  10  and the intermediary image bearing member  15  can be installed in the installing part IP of the main body housing  1 . In this case also, the installing mechanism can be advantageously configured such that either of at least two intermediary image bearing members different from each other in size can be installed in the installing part IP of the main body housing  1 . Here also, intermediary image bearing members being different in size includes three cases, one case in which widths of the intermediary image bearing members in a direction perpendicular to surface moving directions of the intermediary image bearing members are different from each other, another case in which circumferential lengths of the intermediary image bearing members are different from each other, and still another case in which both of widths of the intermediary image bearing members in a direction perpendicular to surface moving directions of the intermediary image bearing members and circumferential lengths of the intermediary image bearing members are different from each other. In this case also, the configurations described above with reference to  FIG. 1  thorough  FIG. 19  may be adopted for the installing mechanism, respectively. 
   Further, when the configurations illustrated in  FIG. 15  and  FIG. 16  are adopted in the image forming apparatuses illustrated in  FIG. 20 through 28 , the transfer belt  115  used for transferring a visible image onto only one side of a recording medium P may be configured, for example, as illustrated in  FIG. 29  relative to an endless belt spanned around the supporting rollers  75 ,  77  and  78 , and the supporting roller  75  may be configured to function as a transfer roller. When a recording medium P is conveyed between the transfer belt  115  and the intermediary transfer member  3 , a transfer voltage having a polarity opposite to that of the toner of a superimposed visible image on the intermediary transfer member  3  is applied to the supporting roller  75  so that the visible image on the intermediary transfer member  3  can be transferred onto one side of the recording medium P. 
   Further, in place of applying a transfer voltage to the supporting roller  75  illustrated in  FIG. 15 ,  FIG. 16  and  FIG. 29 , a transfer voltage having a same polarity as the charging polarity of the toner on the intermediary transfer member  3  may be applied to the supporting roller  74  for the intermediary transfer member  3  so that a visible image on the intermediary transfer  3  is transferred onto an intermediary image bearing member or a recording medium. This also applies to the configurations illustrated in  FIG. 1 ,  FIG. 6 ,  FIG. 19 ,  FIG. 20 ,  FIG. 33 ,  FIG. 25  and  FIG. 26 . Namely, a transfer voltage is applied to the supporting roller  5  for the intermediary transfer member  3  so that a visible image is transferred onto an intermediary image bearing member or a recording medium. 
   Each of the above-described configurations may be appropriately combined with each other to configure an image forming apparatus. In particular, each of the configurations illustrated in and described with reference to  FIG. 1  through  FIG. 19  may be appropriately adopted in the image forming apparatuses configured such that the transfer device  10  and the intermediary image bearing member  15  can be exchanged with each other. 
   Furthermore,  FIG. 30  through  FIG. 33  illustrate another example of the receiving and guiding mechanism configured to receive and guide any of a plurality of intermediary image bearing members that are different in size to be installed in and removed from the installing part IP of the main body housing  1 , thereby enabling exchanging an installed intermediary image bearing member with another one.  FIG. 30  illustrates a state that a belt unit  640  including the intermediary image bearing member  15  that is short in circumferential length is installed in the main body housing  1 , and  FIG. 31  illustrates a state that another belt unit  640 A including the intermediary image bearing member  15 A that is long in circumferential length is installed in the main body housing  1 .  FIG. 32  illustrates the belt units  640  and  640 A removed from the installing part IP of the main body housing  1  and a part of the main body housing  1  from which the belt units  640  and  640 A have been removed, and  FIG. 33  illustrates respective outer appearences of the belt units  640  and  640 A and the main body housing  1 . 
   In this example, as illustrated in  FIG. 33 , the belt units  640  and  640 A are directly installed in the installing part IP of the main body housing  1 . Specifically, guide grooves  400 , serving as the receiving and guiding mechanism of the present invention, are provided to portions of the part of the main body housing  1  forming the installing part IP, and protrusions  410  and  410 A of the belt units  640  and  640 A are engaged with the guide grooves  400  of the main body housing  1 , respectively, and by being pushed substantially in a horizontal direction, the belt units  640  and  640 A are guided by the guide grooves  400  to be installed in the installing part IP of the main body housing  1 , respectively. The belt units  640  and  640 A are removed from the installing part IP of the main body housing  1  by being pulled in the reverse direction, respectively. The width of the opening part of the installing part IP of the main body housing  1  when viewed from the side of the apparatus where the operation panel OP is provided is substantially the same as that of the width of the main body housing  1 . Therefore, the belt units  640  and  640 A can be easily installed in and removed from the installing part IP of the main body housing  1 . Further, when the belt units  640  and  640 A are installed in the installing part IP of the main body housing  1 , the front sides of the covers  660  and  660 A of the belt units  640  and  640 A (when viewed from the side of the apparatus where the operation panel OP is provided) and front portions of both sides thereof are exposed, respectively, so that the covers  660  and  660 A of the belt units  640  and  640 A are formed in different shapes from those of the belt units  64  and  64 A, respectively. Specifically, the covers  640  and  640 A include protruded parts  661 ,  662  and  661 A,  662 A, respectively. In the examples described earlier, the cleaning device  32  is not included in the belt units  64  and  64 A. However, in this example, the cleaning device  32  is integrally included in each of the belt units  64  and  64 A, so that the operation of installing in and removing from the main body housing  1  the belt units  64  and  64 A is simplified. The knobs  39  and  39 A that are grabbed when installing in and removing the belt units  64  and  64 A from the main body housing  1  are provided to the front sides of the covers  660  and  660 A, respectively. In this example, the grooves  400  are provided to the main body housing  1  and the protrusions  410  and  410 A are provided to the belt units  640  and  640 A. However, it may be configured such that grooves are provided to the belt unit  640  and  640 A and protrusions are provided to the main body housing  1 . 
   It is needless to say that the configurations described immediately above can be appropriately adopted in the image forming apparatuses described earlier, including those configured such that the transfer device  10  and the intermediary image bearing member  15  can be exchanged with each other. 
   Furthermore, in the above description, a plurality of intermediary image bearing members that are different in size from each other, and a combination of an intermediary image bearing member and a transfer device are taken as examples of two devices having different functions either of which can be installed in the installing part IP of the main body housing  1  of the image forming apparatus of the present invention. However, devices that can be installed in the installing part IP of the main body housing  1  are not limited to those described herein as these examples. By appropriately configuring the installing part IP of the main body housing  1  or the receiving and guiding mechanism such that either of at least two devices having different functions can be installed in the installing part IP, the image forming apparatus can be changed from one having a certain function to another one having a different function. 
   The present invention can be applied to other image forming apparatuses than printers, for example, copying machines, facsimile machines, and multi-function machines having at least two of printing, facsimile and copying functions. 
   Numerous additional modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the present invention may be practiced otherwise than as specifically described herein. 
   The present application claims priority and contains subject matter related to Japanese Patent Applications No. 2001-385316, NO. 2001-395508, and NO. 2002-324421 filed in the Japanese Patent Office on Dec. 18, 2001, Dec. 26, 2001 and Nov. 7, 2002, respectively, and the entire contents of which are hereby incorporated by reference.