Patent Publication Number: US-7917057-B2

Title: Image forming apparatus with a front cover having a release member disposed to avoid incorrect operation

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image forming apparatus having a front cover in a freely openable/closable manner, which opens, for example, when a treatment for paper jam is carried out, at an anterior surface of the image forming apparatus. 
     2. Description of the Related Art 
     In an image forming apparatus such as a printer, a copier, or a facsimile machine, a sheet is conveyed by a sheet conveying section, and an image is formed on the sheet in an image forming unit. For example, in a case in which an image forming apparatus is an electrophotographic system image forming apparatus commonly used for offices and the like, at the time of paper feeding to send away a sheet stacked in a paper feed cassette, or at the time of fixing a transferred toner image transferred onto a sheet by heating and pressuring it, sheet jam is easy to be caused. Therefore, in an image forming apparatus, an opening and closing cover in an openable/closable manner is provided to a front surface or a left or right side surface, and in a case of paper jam, the opening and closing cover is opened to exposure a part of a sheet conveying path, which makes it possible to easily remove the jammed paper. 
     The opening and closing cover is usually locked with the image forming apparatus main body with a lock member. By releasing this lock member by pushing or pulling release members such as levers or buttons by an operator (a user, a service man, or the like), the opening and closing cover is opened. 
     Conventionally, such a release member is, in order for anyone to easily find it when paper jam is caused, generally disposed at a relatively prominent portion of a front surface side or a side surface of the image forming apparatus, and so as to be easy to operate, for example, as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 2002-351282. 
     However, according to the conventional art, because the release member is disposed so as to be prominent and easy to operate for an operator, as a first problem, there is the risk that, for example, when the operator takes a sheet after image formation out of a paper discharge tray, the operator touches the release member by mistake, or touches the release member without discretion, to carry out an incorrect operation. Further, a second problem that the release member is disposed at a place easy to be viewed so as to be prominent from the operator, which spoils the beauty thereof, will be brought about. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide an image forming apparatus having a release member which is disposed at a place hard to be viewed from an operator operating the image forming apparatus, and hard to be operated in error, but is easy to operate in case of need of operation. 
     An image forming apparatus according to an aspect of the present invention to achieve the object, comprising: an image forming unit which forms an image on a sheet; a sheet conveying section which conveys the sheet on which the image is formed; an apparatus main body in which the image forming unit and the sheet conveying section are installed; an operating unit to input operational information; a front cover to which the operating unit is mounted to be supported in a freely openable/closable manner by the apparatus main body, the front cover is capable of changing in position between a closed position to cover an anterior surface side of the apparatus main body and an open position to expose a part of the apparatus main body; and a retention mechanism which includes a lock member and a release member, the retention mechanism retains the front cover at the closed position with respect to the apparatus main body, wherein the lock member is a member which is supported by one of the apparatus main body and the front cover so as to be capable of making an engagement with and being detached from the other one, and the release member is a member to release engagement of the lock member, and the release member is installed at a region located posterior to the operating unit and a region facing a back surface side as viewed from an opening and closing direction of the front cover. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an overall perspective view of an image forming apparatus according to a first embodiment of the present invention, when it is viewed from a diagonally upward left front direction. 
         FIG. 2  is a perspective view of the image forming apparatus which is viewed from a diagonally upward left rear direction. 
         FIG. 3  is a sectional view schematically illustrating an internal structure of the image forming apparatus. 
         FIG. 4  is a perspective view of a front cover. 
         FIG. 5  is a view of an upper side portion of the front cover in a state in which a left panel is detached, when it is viewed from the left side. 
         FIG. 6  is a view for explanation of a state in which a lock member is operated by operating a release lever serving as a release member from the state illustrated in  FIG. 5 . 
         FIG. 7  is an enlarged perspective view for explanation of structure members of the lock member according to the first embodiment. 
         FIG. 8  is a perspective view illustrating the structure members of the lock member which are taken out. 
         FIGS. 9A and 9B  are schematic diagrams for explanation of the front cover and a conveying unit, and  FIG. 9A  is a diagram illustrating closed positions (closed states) of the both, and  FIG. 9B  is a diagram illustrating open positions (open states) thereof. 
         FIG. 10  is a side view illustrating an image forming apparatus according to a second embodiment of the present invention. 
         FIG. 11  is a perspective view for explanation of a shaft member, a hook member, and a cam follower portion at the side of the front cover, and an engaging member at the side of an apparatus main body in the lock member. 
         FIG. 12  is a perspective view for explanation of a catching concave portion and a cam portion formed at an entrance thereof at the side of the conveying unit. 
         FIG. 13  is a perspective view for explanation of a state in which the shaft member is brought to make an engagement with the catching concave portion. 
         FIGS. 14A to 14C  are operational explanatory diagrams schematically illustrating statuses in which the shaft member is brought to make an engagement with the catching concave portion, and the hook member is brought to make an engagement with the engaging member. 
         FIG. 15  is a left side view illustrating a state of an opening and closing aid mechanism according to a third embodiment of the present invention, when the front cover is disposed at a closed position. 
         FIG. 16  is a perspective view illustrating a state of the opening and closing aid mechanism when the front cover is disposed substantially an intermediary position between the closed position and an open position. 
         FIG. 17  is a left side view illustrating a state of the opening and closing aid mechanism when the front cover is disposed at the open position. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. Note that, in the respective drawings, members and the like denoted by same reference numerals respectively have same structures, and overlapping descriptions thereof will be appropriately omitted. Further, in the respective drawings, members and the like which are not required for explanation are appropriately omitted to be illustrated. 
     First Embodiment 
     An entire structure of an image forming apparatus  1  according to a first embodiment will be described with reference to  FIGS. 1 to 4 . Note that, in  FIGS. 1 to 3 , up and down, front and back, and right and left directions with respect to the image forming apparatus  1  are indicated by arrows. In  FIG. 4 , a perspective view of a front cover  3  in a state of being detached from the sate of  FIG. 1  is shown. However, a manual paper feed tray  16  which will be described later is detached in  FIG. 4 . 
     Here, as the image forming apparatus  1 , a printer, a copier, a facsimile machine, and a multifunction machine thereof, or the like may be cited. However, in the following description, an example in a case in which the image forming apparatus  1  is a printer will be described. The image forming apparatus  1  is a full-color printer of four colors that employs an electrophotographic system, an intermediate transfer system, and a tandem system. 
     A structure in which the entire image forming apparatus  1  is viewed from the outside will be described with reference to  FIGS. 1 and 2 . The image forming apparatus  1  includes a substantially box-shaped (rectangular parallelpiped form) apparatus main body  2  of the image forming apparatus, and a front cover  3  supported in an openable/closable manner freely by the apparatus main body  2 . 
     With respect to the apparatus main body  2 , its anterior surface is covered with the front cover  3 , and its left side and right side are covered with a left face panel  4  and a right face panel  5  which are made of synthetic resin. At a posterior surface of the apparatus main body  2 , a part  7  of a sheet metal structuring a main body frame  6  is exposed as a structural object. At a superior surface of the apparatus main body  2 , its anterior part is covered with the front cover  3 , and a portion from an intermediate part to a posterior part is covered with a paper discharge tray  10  having a sheet loading surface  8  with a rising rear. 
     The front cover  3  includes a front face panel  11  located on an outer side, and a wall surface located at an inner side, which structures a part of a sheet conveying section  24  which will be described later. The front face panel  11  is composed of an anterior panel  12 , an upper panel  13  with a rising rear which continues to the upper end the anterior panel  12 , and inverted “L” left panel  14  and right panel  15 , which are integrally structured. 
     The rectangular manual paper feed tray  16  is installed in the anterior panel  12 . The manual paper feed tray  16  has a center of swinging at its lower end side, and at a closed position in a standing position illustrated in  FIG. 1 , the manual paper feed tray  16  structures a part of the anterior panel  12 , and at an open position with a dropping rear in a state in which its upper end side is pulled out forward (not shown), the manual paper feed tray  16  serves as a paper feed stand on which sheets are placed on its superior surface. 
     An operation panel (operating unit)  17  to receive inputs of instructional information is disposed on the upper panel  13 . The operation panel  17  is disposed so as to be turned substantially upward at a gentle slant with a slightly higher rear end side in order for an operator (for example, a user) standing in front of the image forming apparatus  1  to operate, to be easy to see it. A touch-panel type liquid crystal display unit and various buttons are disposed in the operation panel  17 , and it is possible for the operator to carry out various information operations with respect to the image forming apparatus  1  through this operation panel  17  in a state of standing in front of the image forming apparatus  1 . The left panel  14  and the right panel  15  are installed so as to respectively cover a part of the sheet conveying section  24  structured inside the front cover  3  from the left and the right. 
     The front cover  3  is, as will be described later, supported swingablly at its lower end side by the apparatus main body  2 , and the entire front cover  3  is formed in a freely openable/closable manner such that its upper end side is opened so as to be separated away from the apparatus main body  2  at the open position. Release levers (release members)  80 , which will be described later, to be operated at the time of opening the front cover  3 , are installed at an inner side (right side) portion at the upper end side and the rear end side of the left panel  14  and at an inner side (left side) portion at the upper end side and the rear end side of the right panel  15  as illustrated in  FIG. 2 . Note that the release levers  80  will be described later in detail. 
     With respect to the front cover  3  having the above-described structure, it is possible for the operator to carry out a treatment for paper jam (a work for removing jammed sheets) after opening the front cover  3  toward the front side from the front surface side of the image forming apparatus  1 . Moreover, as will be described later, it is possible to attache and detach a paper feed cassette  25  in which sheets to be objects on which images are formed are stacked, from the front surface side in the same way. In this way, it is possible for the operator to carry out all the entire operations of the image forming apparatus  1  by use of the operation panel  17 , a treatment for the paper jam when paper jam is caused, supplement of sheets to the paper feed cassette  25  when sheets are run out, and the like, from the front surface side of the image forming apparatus  1 . 
     Next, an internal structure of the image forming apparatus  1  will be described with reference to  FIG. 3 . In the image forming apparatus  1 , a sheet housing unit  20 , a substrate housing unit  21 , an image forming unit  22 , a toner supply unit  23 , and the paper discharge tray  10  are provided in order from the lower side to the upper side of the apparatus main body  2 . Further, the sheet conveying section  24  is provided between the anterior side of the apparatus main body  2  and the front cover  3 . 
     The paper feed cassette  25  is installed in the sheet housing unit  20 . The paper feed cassette  25  accommodates plural pieces of sheets in a stacked state, and the leading end side of the sheets in the stacked state (the right side in the drawing) are biased upward by a lift plate  26  installed at a bottom portion thereof. In accordance therewith, a sheet at the top in the paper feed cassette  25  is fed by a pickup roller  27  of the sheet conveying section  24  which will be described later, and further, lap-feeding is prevented by a feeding roller  28  and a retarding roller  30 , and only one sheet is fed downstream. This paper feed cassette  25  is capable of being detached and attached from the front surface side of the image forming apparatus  1 . The substrate housing unit  21  is installed above the paper feed cassette  25 . 
     A substrate and electric components (not shown) to control the entire image forming apparatus  1  are installed in the substrate housing unit  21 . The image forming unit  22  is installed above the substrate housing unit  21 . 
     An intermediate transfer belt  31 , and four (four-color) image forming stations having similar structures to each other, i.e., an yellow (Y) image forming station  32 , a magenta (M) image forming station  33 , a cyan (C) image forming station  34 , and a black (B) image forming station  35  along a rotational direction of the intermediate transfer belt  31  (a direction of an arrow R  31 ) are installed in the image forming unit  22 . 
     The yellow image forming station  32  has a photosensitive drum  36 , and a charging device  37 , an exposure device  38 , a development device  40 , a primary transfer roller  41 , a drum cleaner  42 , and the like which are installed along the rotational direction (the arrow direction) at the periphery of the photosensitive drum  36 . The photosensitive drum  36  is driven to rotate at a predetermined processing speed in the arrow direction. After the outer peripheral surface of the photosensitive drum  36  is charged uniformly to have predetermined polarity and electric potential by the charging device  37 , electric charges on an exposed portion are eliminated by exposure of the exposure device  38  according to image information transmitted from a personal computer (not shown) or the like, which forms an electrostatic latent image. This electrostatic latent image is developed as a toner image so as to adhere toner in a developer onto it by the development device  40 . 
     The toner image formed on the surface of the photosensitive drum  36  is transferred onto the intermediate transfer belt  31 . The intermediate transfer belt  31  is bridged between a driving roller  43  and a driven roller  44 , and is rotated in the direction of the arrow R 31  by rotation in the arrow direction of the driving roller  43 . The yellow toner image formed on the photosensitive drum  36  is primarily transferred on the intermediate transfer belt  31  by the primary transfer roller  41  in a primary transfer section T 1 . After the primary transfer of the toner image, toner remaining on the surface of the photosensitive drum  36  (primary transfer residual toner) is removed by the drum cleaner  42 . 
     The remaining three-color (cyan, magenta, black) image forming stations  33 ,  34 , and  35  as well have structures similar to that of the above-described yellow image forming station  32 . On the surfaces of the photosensitive drums  36  of these image forming stations  33 ,  34 , and  35 , toner images of the respective colors of cyan, magenta, and black are formed to be sequentially primarily transferred on the intermediate transfer belt  31  in the same way. In this way, the four-color toner images are superimposed on the intermediate transfer belt  31 . The four-color toner images on the intermediate transfer belt  31  are collectively secondarily transferred onto a sheet conveyed by the sheet conveying section  24  which will be described later, by a secondary transfer roller  45  in a secondary transfer section T 2 . Toner remaining on the surface of the intermediate transfer belt  31  after the secondary transfer (secondary transfer residual toner) is removed by a belt cleaner  46  disposed in the vicinity of the driven roller  44 . The toner supply unit  23  is provided above the image forming unit  22 . 
     Four toner containers in which toners of the respective colors are respectively housed separately, i.e., yellow, magenta, cyan, and black toner containers  47 ,  48 ,  50 , and  51  are installed in the toner supply unit  23 . Density sensors (not shown) to sense a concentration of toner (a ratio by weight of toner/developer) are installed in the development devices  40  of the respective colors. When the concentration sensors sense that amounts of toners in the development devices  40  get less than predetermined values, toners are replenished to the development devices  40  of the respective colors from the respective color toner containers  47 ,  48 ,  50 , and  51 . The paper discharge tray  10  is provided above the toner supply unit  23 . 
     The paper discharge tray  10  is formed so as to cover the superior surface of the apparatus main body  2 . An intermediate part in an anteroposterior direction of the paper discharge tray  10  is at a slant so as to locate its rear end side above, and the rear end side of the paper discharge tray  10  is formed to be flat so as to continue to the intermediate part. Sheets discharged backward from a sheet discharge opening  55  of the sheet conveying section  24  which will be described next, are loaded on the sheet loading surface  8  on the superior surface of the paper discharge tray  10 . 
     The sheet conveying section  24  is, in the present embodiment, provided between the anterior side of the apparatus main body  2  and the front cover  3 . The sheet conveying section  24  has a sheet conveying path  52  (a first sheet conveying path) to guide a sheet from a lower side toward an upper side, a sheet reconveying path  53  (a second sheet conveying path), which is installed at a side anterior to the sheet conveying path  52 , to guide a sheet from an upper side toward a lower side, and a manual feeding section  54 . Note that, in the present embodiment, the sheet reconveying path  53  is a sheet conveying path directly opened by opening the front cover  3 , which will be described later. Further, the sheet conveying path  52  is a sheet conveying path indirectly opened via a conveying unit  73 , which will be described later, by opening the front cover  3 . 
     The sheet conveying path  52  orients upwardly from the vicinity of the above-described feeding roller  28 , and extends upward so as to gently curve backward in a convex manner, and extends upward so as to turn a curving direction forward in the vicinity of the intermediate transfer belt  31 , and extends diagonally upward and backward, to reach the sheet discharge opening  55 . The sheet conveying path  52  is composed of a rear side guide  52   a  and a front side guide  52   b  facing each other, and a part of the front side guide  52   b  is formed in the conveying unit  73  which will be described later. 
     At the sheet conveying path  52 , the pickup roller  27 , the feeding roller  28 , the retarding roller  30 , a pair of conveying path rollers  56 , a pair of registration rollers  57 , the secondary transfer roller  45  interposing the above-described intermediate transfer belt  31  between it and the driving roller  43 , a pair of fixing rollers  58 , a pair of conveying rollers  60 , a switching flapper  61 , a pair of paper discharge rollers  62 , and the like are installed in order from the lower side. The pair of fixing rollers  58  includes a fixing roller  63  having a heater (not shown) built-in and a pressure roller  64  brought into contact with the fixing roller  63  to structure a fixing nip portion therebetween. 
     Only one sheet fed from the paper feed cassette  25  by the pickup roller  27 , the feeding roller  28 , and the retarding roller  30  is conveyed by the pair of conveying rollers  56  and the pair of registration rollers  57 . The four-color toner images on the intermediate transfer belt  31  are collectively secondarily transferred on this paper in the secondary transfer section T 2 , and when this sheet pass through the fixing nip portion, the sheet is heated and pressed to fix the toner image on its surface. The sheet on which the toner image has been fixed is guided to the lower surface of the switching flapper  61  by the pair of conveying rollers  60  to be conveyed to the pair of paper discharge rollers  62 . The sheet is discharged backward from the sheet discharge opening  55  facing the back surface side by the pair of paper discharge rollers  62 , to be loaded on the sheet loading surface  8  of the paper discharge tray  10 . Note that, in  FIG. 3 , a state in which a sensor flag  65  of a paper discharging sensor disposed directly downstream of the pair of paper discharge rollers  62  is operated by a sheet P in the process of being discharged. 
     The sheet reconveying path  53  extends at a slant toward the anterior side from slightly above the sheet discharge opening  55 , and extends at a steep slant backward so as to gently curve, and joins together with the sheet conveying path  52  so as to curve downward in a convex manner at the lower end portion. The sheet reconveying path  53  is composed a rear side guide  53   a  and a front side guide (guiding portion)  53   b  facing each other. A part of the rear side guide  53   a  is formed in the conveying unit  73  which will be described later. Further, most of the front side guide  53   b  is built into the inner side (rear end side) of the face panel  11 , and structures the front cover  3  along with the face panel  11 . At the sheet reconveying path  53 , a pair of reverse rollers  66 , a switching flapper  61 , first, second, third, and fourth pairs of reconveying rollers  67 ,  68 ,  70 , and  71  are installed in order from the upper side which is the upper stream side at the time of reconveying a sheet. 
     When an image is formed on both surfaces of a sheet, the switching flapper  61  is switched to a position shown by the chain double-dashed line. The sheet on which the toner image is fixed on its surface is conveyed to the pair of conveying rollers  60  to be conveyed along the superior surface of the switching flapper  61 , and is further conveyed backward by the pair of reverse rollers  66 . Before the trailing end of the sheet passes through the pair of reverse rollers  66  after passing through the pair of conveying rollers  60 , the pair of reverse rollers  66  is reversely rotated. In accordance therewith, the sheet is conveyed downward by the first to fourth pairs of reconveying rollers  67 ,  68 ,  70 , and  71  to be conveyed to the sheet conveying path  52 . Then, the toner image is transferred and fixed on the reverse surface of the sheet in the same way as the surface, and the sheet is discharged backward from the sheet discharge opening  55  to be loaded on the sheet loading surface  8  of the paper discharge tray  10 . Note that a manual feeding roller  72  is installed directly in front of the pair of conveying rollers  56  in the sheet conveying section  24 , and the manual paper feed tray  16  structuring a part of the front cover  3  is pulled out in a closed state, and it is possible to feed sheets set on the manual paper feed tray  16  toward the side of the pair of conveying rollers  56 . 
     As described above, the sheet conveying section  24  is composed of the conveying unit  73  capable of opening and closing partially. The conveying unit  73  is disposed between the sheet conveying path  52  and the sheet reconveying path  53 . The conveying unit  73  includes a part of the front side guide  52   a  of the sheet conveying path  52  and a part of the rear side guide  53   a  of the sheet reconveying path  53 , one roller  57   a  of the pair of registration rollers  57 , the secondary transfer roller  45 , and respective one side rollers  70   a  and  71   a  of the third and fourth pairs of reconveying rollers  70  and  71 , which are integrally structured. 
     The conveying unit  73  is supported swingablly at its lower end side by the apparatus main body. The conveying unit  73  is interposed between the front cover  3  disposed at the closed position and the anterior surface side of the apparatus main body  2 , and is supported in a freely openable/closable manner by the apparatus main body  2  so as to be capable of changing in position between a closed position to structure a part of the sheet conveying path  52  and a part of the sheet reconveying path  53 , and an open position to open a part of the sheet conveying path  52  and the sheet reconveying path  53  according to opening of the front cover  3 . 
     Next, the front cover  3  will be described in detail with reference to  FIGS. 4 to 9 .  FIG. 4  is a perspective view of the front cover  3  in a state of being detached from the image forming apparatus  1  illustrated in  FIG. 1 . However, in  FIG. 4 , a state in which the manual paper feed tray  16  is detached is illustrated.  FIG. 5  is a view of an upper side portion of the front cover  3  in a state in which the left panel  14  (refer to  FIG. 1 ) is detached, which is viewed from the left side.  FIG. 6  is a view illustrating a state in which the release lever  80  is operated from a state illustrated in  FIG. 5 .  FIG. 7  is a perspective view illustrating the release lever  80  and a lock member  81  (lock member assembly). A retention mechanism  82  is composed of the release lever  80  and the lock member  81 .  FIG. 8  is a perspective view illustrating the lock member  81  which is taken out. Further,  FIGS. 9A and 9B  are schematic views for explanation of the front cover  3  and the conveying unit  73 , and  FIG. 9A  is a view illustrating closed positions A 1  and B 1  of the front cover  3  and the conveying unit  73 , and  FIG. 9B  is a view illustrating open positions A 2  and B 2  thereof. Note that, in  FIGS. 9A and 9B , a left side plate  83  and a right side plate  84  structuring a part of the main body frame  6  of the apparatus main body  2  are illustrated by the chain double-dashed lines. The left side plate  83  is disposed so as to cover the left side surface of the conveying unit  73  and the front cover  3 , and the right side plate  84  is disposed so as to cover the right side surface of the conveying unit  73  and the front cover  3 . 
     As illustrated in  FIG. 4 , the front cover  3  has a pivot point  85  at the lower end, and the pivot point  85  is supported pivotally by the apparatus main body  2 . With respect to the apparatus main body  2 , as illustrated in  FIGS. 9A and 9B , portions extending forward of the left side plate  83  and the right side plate  84  of the main body frame  6  made of plate cover the parts of the left side surface and the right side surface of the front cover  3  and the left side surface and the right side surface of the conveying unit  73 . 
     A shaft  86  is provided to project at the lower ends of the anterior end sides of the left side plate  83  and the right side plate  84 . The pivot point  85  of the front cover  3  is supported pivotally by the shaft  86 . Hereby, the front cover  3  is capable of changing in position between the closed position A 1  illustrated in  FIG. 9A  and the open position A 2  illustrated in  FIG. 9B . The above-described conveying unit  73  as well has a pivot point (not shown) at the lower end side, and is supported pivotally by a shaft (not shown) provided to project from the left side plate  83  and the right side plate  84  at the side of the apparatus main body  2 . Hereby, the conveying unit  73  is capable of changing in position between the closed position B 1  illustrated in  FIG. 9A  and the open position B 2  illustrated in  FIG. 9B . 
     In the present embodiment, with respect to the conveying unit  73 , there is no need for the operator to carry out opening and closing operations positively in particular, and when the front cover  3  is opened, the conveying unit  73  is opened according to that, and when front cover  3  is closed, the front cover  3  is closed according to that. Note that the front cover  3  and the conveying unit  73  stop in a state of retaining appropriate slope positions, as illustrated in  FIG. 9B , by stoppers, springs, and the like provided at the side of the apparatus main body  2 . 
     When the front cover  3  is opened, the operator operates the release lever  80  of the retention mechanism  82 . As illustrated in  FIGS. 5 to 8 , the retention mechanism  82  includes the release lever  80  (release member) and the lock member  81  operated by the release lever  80 . The lock member (lock member assembly) has a slide member  86 , a rotary member  87 , a connecting member  88 , and a hook  90 . 
     The release lever  80  is formed to be vertically long, and a circular arc shaped fulcrum  91  is provided to project at the lower end side. The fulcrum  91  is brought into contact with a contact portion  92  (illustrated by the dashed-two doted line) serving as a part of the front cover  3 . The release lever  80  is capable of moving its upper end side in a substantially anteroposterior direction centering on the fulcrum  91 . On the back surface sides at the upper end sides of the release levers  80 , as illustrated in  FIG. 2 , in order for an operator to easily get his or her fingers caught in, circular buttons  93  are formed. The buttons  93  face the back surface side, which cannot be viewed from the operator operating the operation panel  17  at the front surface side (the opening and closing direction of the front cover  3 ) of the image forming apparatus  1 . 
     A measure from the fulcrum  91  up to the center of the button  93  is set such that, when an ordinary adult gently puts together four fingers other than his or her first finger, and puts his or her index finger onto the center of the button  93 , his or her little finger at the lower side touches a portion above the fulcrum  91 . Moreover, the vicinity of a portion corresponding to the fulcrum  91  at the back surface side of the release lever  80  gently curves backward in a convex manner. Thereby, except for a case in which the operator operates an extremely lower end side of the release lever  80 , even when any position thereof is operated, the upper end side having the button  93  moves forward. 
     A pressing portion  94  is provided to project at the anterior surface side at the upper end side of the release lever  80 , and the pressing portion  94  is brought into contact with a base end portion  95  of the slide member  86 . In the present embodiment, the release levers  80  are disposed so as to face the back surface side at positions posterior to the operation panel  17 , and at the inner side (right side) at the upper end side and the rear end side of the left panel  14  (refer to  FIG. 2 ), and at the inner side (left side) at the upper end side and the rear end side of the right panel  15 . The positions correspond to the left side and the right side of the peripheral portion of the sheet discharge opening  55 . To further describe, because the sheet discharge opening  55  is structured to face the back surface side so as to project upward from the paper discharge tray  10  on the superior surface of the apparatus main body  2  at the anterior part at the superior surface of the apparatus main body  2 , it is possible for the operator located at the front surface side of the image forming apparatus  1  to easily operate it at the time of operating the release levers  80  positively with apparent intention to operate it. 
     The slide member  86  is formed into a rod-like shape anteroposteriorly long, and as illustrated in  FIG. 7 , the slide member  86  is installed so as to be anteroposteriorly slidable by being interposed between a left slide guide  96  and a right slide guide (not shown) which are anteroposteriorly provided to project upward. The slide member  86  is restricted on its vertical positions so as to be not lifted by the upper panel  13  (refer to  FIGS. 1 and 2 ). The slide member  86  is brought into contact with the pressing portion  94  of the release lever  80  at its base end portion (first end portion)  95 , and brought into contact with the rotary member  87  which will be described next at its leading end portion (second end portion)  97 . 
     The rotary member  87 , as illustrated in  FIGS. 5 to 7 , its shape viewed from the left is formed into a substantially fan shape, and is made rotatable centering on the shaft  98  integrally structured therewith. The rotary member  87  has a contacting portion  100  at the rear end side, and a leading end portion  97  of the slide member  86  is brought into contact with the contacting portion  100 . Further, the rotary member  87  has a catching portion  101  at the anterior end side, and a base end portion  102  of the connecting member  88  which will be described next is brought to make an engagement with the catching portion  101 . The rotary member  87  is configured to pull the base end portion  102  (first connecting portion) of the connecting member  88  diagonally downward left when being rotated in a clockwise direction in  FIG. 5  centering on the shaft  98 . 
     The connecting member  88  is formed into a belt shape with flexibility, and the base end portion  102  is brought to make an engagement with the rotary member  87 , and an intermediate part  103  extends forward and gently curves along the front cover  3 , and further extends downward to make the leading end portion  104  (second connecting portion) connect to the base end portion  105  of the hook  90 . Note that, in  FIG. 8 , the connecting member  88  in a state in which the engagement with the rotary member  87  is released is illustrated by the chain double-dashed line. The connecting member  88  is guided by being fitted into a groove  106  (refer to  FIG. 7 ) formed in the front cover  3 . 
     The base end portion  105  of the hook  90  is connected to the leading end portion  104  of the connecting member  88 , and the intermediate part  106  thereof is supported pivotally by a shaft  109  at the side of the front cover  3 . The upward catching concave portion  108  is formed in the leading end portion  107  of the hook  90 , and an inclined portion  110  is formed at a further leading end side of the catching concave portion  108 . A catching convex portion  111  (a predetermined engaging member) with which the catching concave portion  108  of the hook  90  is brought to make an engagement and detached is provided to project on the left side plate  83  of the apparatus main body  2  illustrated in  FIGS. 9A and 9B . Note that the hook  90  is biased in a clockwise direction in  FIG. 5  by a biasing member (for example, a torsion spring), and the hook  90  brought to make an engagement with the catching convex portion  111  is biased by the biasing member so as to retain the engaged state with the catching convex portion  111 . 
     Note that a stopper (not shown) to restrict a pivoting range (moving range) of the hook  90  is provided to the front cover  3 , and a position of the hook  90  at the time of releasing the engagement, i.e., at the time of opening the front cover  3  is restricted thereby. The stopper is set to be located such that, when the front cover  3  in an open state is gradually closed, the inclined portion  110  of the hook  90  touches the catching convex portion  111  to once retreat, and the catching concave portion  108  is brought to make an engagement with the catching convex portion  111 . Note that, in  FIGS. 5 and 6 , timing belts  120  and  121  to drive the first, second, and third pairs of reconveying rollers  67 ,  68 , and  70  (refer to  FIG. 3 ) are illustrated. 
     The release levers  80  are installed at the both of the left end side and the right end side of the front cover  3 . Hereby, as illustrated in  FIG. 8 , a right slide member  112  and a part  113  of the rotary member are installed at a position corresponding to the right release lever  80 , and the part  113  is integrally structured with the rotary member  87  via the shaft  98 . Moreover, a hook  114  having a catching concave portion  115  is installed at the right side, and is fixed to the shaft  109  integrated with the left hook  90 . Note that a catching convex portion (not shown) which is similar to the catching convex portion  111  of the left side plate  83  is installed at a position corresponding to the catching concave portion  115  of the right hook  114 . In the present embodiment, because the lock member  81  is structured as described above, provided that at least one of the left and right release levers  80  is operated, it is possible to release locking of the hooks  90  and  114  with respect to the catching convex portion  111  and the like to open the front cover  3 . 
     Here, the respective members structuring the above-described retention mechanism  82 , i.e., the release lever  80 , the slide member  86 , the rotary member  87 , the connecting member  88 , and the hook  90  may be formed of, for example, synthetic resin. Moreover, in the present embodiment, because the connecting member  88  and the hook  90  are formed integrally, it is possible to decrease a number of components of structural members of the retention mechanism  82  by that much. 
     For example, when paper jam is caused in the sheet conveying section  24 , the front cover  3  can be opened as follows. When the operator presses the button  93  of the release lever  80  at least one of the left and right release levers  80  illustrated in  FIG. 2  toward the front side (anterior surface side: in a direction of an arrow a) as illustrated in  FIG. 6 , the pressing portion  94  of the release lever  80  presses and moves the slide member  86  toward the front side (in a direction of an arrow b). Hereby, the leading end portion  97  of the slide member  86  makes the rotary member  87  rotate in a direction of an arrow c. Then, because the base end portion  102  of the connecting member  88  is pulled in a direction of an arrow d, the leading end portion  104  of the connecting member  88  is pulled up in a direction of an arrow e to lift up the base end portion  105  of the hook  90 , which makes the leading end portion  107  of the hook  90  rotate in a direction of an arrow f. 
     Thereby, the engagement of the catching concave portion  108  of the hook  90  with respect to the catching convex portion  111  at the side of the apparatus main body  2  is released. Moreover, it is possible for the operator facing the front cover  3  to open the front cover  3  to change its position to the open position A 2  illustrated in  FIG. 9B  by pulling the vicinity of the release lever  80  of the front cover  3  to the front side. According to this, the conveying unit  73  as well is opened to change its position to the open position B 2 . 
     Accordingly, because the sheet reconveying path  53  between the front cover  3  and the conveying unit  73 , and the sheet conveying path  52  between the apparatus main body  2  and the conveying unit  73  are opened, it is possible for the operator to easily remove the jammed sheet. After the treatment for paper jam, when the operator closes the front cover  3  to return it to the closed position A 1  while pushing the conveying unit  73  with the front cover  3 , the hook  90  of the retention mechanism  82  is brought to make an engagement with the catching convex portion  111  at the side of the apparatus main body  2 , which retains a closed state. 
     In the present embodiment, because the release levers  80  are installed on the plane (portion) located posterior to the operation panel  17  at the upper end side of the front cover  3 , and on the plane (portion) facing the back surface side, it is difficult to view the release levers  80  from the front surface side of the image forming apparatus  1 , where the operator is located at the time of operating the operation panel  17 , which makes it hard to carry out an incorrect operation. Additionally, when the operator makes an attempt to operate the release lever  80  positively with the intention of operating it, it is possible for the operator to easily operate it without moving from the front surface side in particular. 
     Further, because both of the anterior panel  12  and the upper panel  13  of the front cover  3  are formed to be relatively flat, and there are no portions to be grasped particularly at the time of opening the front cover  3 , the operator naturally puts his or her hands on the rear ends of the left panel  14  and the right panel  15  projected upward from the paper discharge tray. Then, when the operator puts his or her right and left first fingers on the left end and the right end at the rear end of the upper panel  17 , the other four fingers are put on the release levers  80 , and in particular, the index fingers are to be hooked on the buttons  93  of the release levers  80 . Then, provided that the buttons  93  are pressed toward the front side, and directly, the left and right hands are withdrawn to the front side, it is possible to open the front cover  3 . Namely, because a direction of operating the release levers  80  by the operator and a direction of opening the front cover  3  are substantially the same, it is possible for the operator to smoothly carry out an operation of the release levers  80  to an operation of opening the front cover  3  as a series of operations along a same direction. 
     The release levers  80  are installed on the datum plane along a substantially vertical direction with respect to a plane (datum plane) perpendicular to the anteroposterior direction. Namely, the buttons  93  of the release levers  80  are directed to the back surface side (substantially right behind). Therefore, the release levers  80  can be hardly viewed with eyes from the front surface side of the image forming apparatus  1 , which does not spoil the beauty thereof. 
     In the present embodiment, the effect that the release levers  80  are directed to the back surface side is not limited to be right behind. Namely, there are cases to be included in which the release levers  80  are disposed on a virtual plane surface disposed at an appropriate angle with respect to the datum plane. For example, even in cases in which a virtual plane surface is at an inclined angle of 15 degrees, 30 degrees, 45 degrees, 60 degrees, or the like with respect to the datum plane, the release levers  80  is made hard to be viewed from the front surface side. In an extreme case, if an inclined angle is less than 90 degrees, in other words, in a case of a virtual plane surface in which a normal line set up on the virtual plane surface has a backward component, the release levers  80  are made to hard to be viewed at least from the front surface side. 
     Second Embodiment 
     In the first embodiment, the explanation has been made with a focus on the places at which the release levers  80  are disposed. In this second embodiment, an embodiment with a focus on a structure for positioning the front cover  3  and the conveying unit  73  in the closed states will be described. 
       FIG. 10  is a side view illustrating an image forming apparatus  1 A according to the second embodiment of the present invention, and a view illustrating a position at which a lock mechanism  280  (corresponding to the lock member assembly) according to the present embodiment is disposed. The lock mechanism  280  (a hook member  283 ) is a member corresponding to the lock member  81  (the hook  90 ) in the first embodiment, and is to position and fix the front cover  3  and the conveying unit  73  to the apparatus main body  2 . In  FIG. 10  and  FIGS. 11 to 14 , portions denoted by the same reference numerals of  FIGS. 1 to 9  used for the explanation of the first embodiment denote the same portions, and descriptions thereof are omitted or simplified. 
     As illustrated in  FIG. 11 , the lock mechanism  280  has an engaging member  281  provided to the side of the apparatus main body  2 , a shaft member  282 , a hook member  283 , and a cam follower portion  284  which are provided to the side of the front cover  3 , and a cam portion  285  provided to the side of the conveying unit  73  as illustrated in  FIGS. 12 and 13 . Note that, among the above-described structure members of the lock mechanism  280 , the structure members other than the shaft member  282  (corresponding to the shaft  109  in the first embodiment; refer to  FIG. 8 ) are installed at the both of the left end side and the right end side of the apparatus main body  2 . However, because those respectively have same structures, only the structure members at the right end side will be described hereinafter, and description of the structure members at the left end side will be omitted. 
     The engaging member  281  is structured so as to project to the inner side from the right side plate  84  structuring a part of the main body frame  6 . The lower end of the engaging member  281  is composed of a curved surface  281   a  in order for a catching portion  283   a  of the hook member  283  brought to make an engagement from the lower side as will be described later, to be easy to engage therewith. This engaging member  281  is a member provided to the side of the apparatus main body  2 , to serve as a reference at the time of positioning the front cover  3 . 
     The shaft member  282 , the hook member  283 , and the cam follower portion  284  are integrally structured in the present embodiment. Thereamong, the shaft member  282  is installed along a horizontal direction, and the both end portions are supported rotatably by the front cover  3 . When the front cover  3  disposed at the open position A 2  (refer to  FIG. 9 ) is closed, the conveying unit  73  is pushed by the front cover  3 . However, in the vicinity of the closed position A 1  of the front cover  3  (the closed position B 1  of the conveying unit  73 ), the shaft member  282  directly pushes the conveying unit  73  to finally carry out positioning thereof. 
     The hook member  283  is provided to face substantially backward at the right end side of the shaft member  282 . The catching portion  283   a  in a concave shape which is opened upward is formed at the upper portion at the rear end side of the hook member  283 . This catching portion  283   a  is, as will be described later, brought to make an engagement with and detached from the engaging member  281  from a substantially lower side by rotation of the shaft member  282 . 
     The cam follower portion  284  is formed at a slightly inner side from the hook member  283  in the shaft member  282 . A planar copying surface  284   a  formed by notching a part of an outer peripheral surface  282   a  of the pillar-type shaft member  282 , is formed in the cam follower portion  284 . The copying surface  284   a  moves so as to follow the cam portion  285  which will be described later. Note that, as illustrated in  FIG. 13 , a notch  286  in a same shape as the cam follower portion  284  is formed at a side opposite to the cam follower portion  284 , i.e., a position rotated by  180  degrees in a direction of the circumference of the shaft member  282 . This notch  286  is to be guided to the lower side portion opposed to one another in an opening portion  288  of the catching concave portion  287  when the shaft member  282  is brought to make an engagement with the catching concave portion  287 . 
     The shaft member  282 , the hook member  283 , and the cam follower portion  284  are integrally structured, and as described above, the shaft member  282  is supported rotatably by the front cover  3 . Accordingly, when force is applied to the cam follower portion  284 , the shaft member  282  is rotated by the force, and the hook member  283  is swung according to the rotation of the shaft member  282 . 
     The cam portion  285  is, as illustrated in  FIGS. 12 and 13 , formed at an entrance of the catching concave portion  287  of the conveying unit  73 . The catching concave portion  287  is formed into a shape with the opening portion  288 , which has a cavity which is more like a circle rather than a semicircle in cross section. An inside diameter L 1  of the catching concave portion  287  is set to be slightly greater than an outside diameter L 2  of the shaft member  282 . The opening portion  288  is structured between the cam portion  285  formed at the upper side and a convex guide portion  290  disposed at the lower side so as to be opposed to the cam portion  285 . Positions at which these cam portion  285  and guide portion  290  are formed in a horizontal direction, i.e., positions along the longitudinal direction of the shaft member  282  are positions corresponding to the cam follower portion  284  and the notch  286  described above. 
     The cam portion  285  is formed into a planar shape at a slant so as to have a higher anterior end side. A width measure L 3  between the cam portion  285  and the guide portion  290  is set to be less than the inside diameter L 1  of the catching concave portion  287  and the outside diameter L 2  of the shaft member  282 , and to be slightly greater than a width measure L 4  between the cam follower portion  284  and the notch  286  in the shaft member  282 . Where, to organize a magnitude relation among L 1  to L 4 , those are, in descending order, the inside diameter L 1  of the catching concave portion  287 , the outside diameter L 2  of the shaft member  282 , the width measure L 3  of the opening portion  288 , and the width measure L 4  between the cam follower portion  284  and the notch  286 , that is L 1 &gt;L 2 &gt;L 3 &gt;L 4 . 
     Because those are set to be such a dimensional relation, when the shaft member  282  is brought to make an engagement with the catching concave portion  287 , only in a case in which a direction of tilt of the cam follower portion  284  and a direction of tilt of the cam portion  285  are the same, the shaft member  282  passes through the opening portion  288  to be able to engaging with the catching concave portion  287 . When the shaft member  282  is rotated at the time of engaging or after engaging, a direction of tilt of the cam follower portion  284  and a direction of tilt of the cam portion  285  are shifted, and therefore, the shaft member  282  does not drop out of the catching concave portion  287 . 
     A convex portion  291  is formed on the boarder between the catching concave portion  287  and the cam portion  285 . With respect to the shaft member  282  at the time of engaging, the hook member  283  is rotated in a direction for engaging with the engaging member  281  by the convex portion  291 . 
     Another catching concave portion  292  is formed on the left side of the catching concave portion  287  along the horizontal direction. This catching concave portion  292  is formed into a substantially semi-circular shape, and a guide portion  293  at a slant in a same direction as the cam portion  285  is provided to the upper end side. The guide portion  293  is to guide the outer peripheral surface of the shaft member  282  at the time of engaging. 
     Operations of the lock mechanism  280 , in particular, operations for engaging the hook member  283  with respect to the engaging member  281  will be described with reference to  FIGS. 9A and 9B ,  FIG. 10 , and  FIGS. 14A to 14C . 
     As described above on the basis of  FIG. 9B , in a state in which the front cover  3  is disposed at the open position A 2 , and the conveying unit  73  is disposed at the open position B 2 , the sheet conveying path  52  and the sheet reconveying path  53  are opened.  FIG. 10  illustrates this state. By moving the front cover  3  at the open position A 2  toward a direction of an arrow R 3   b  of  FIG. 10 , it is possible to make the front cover  3  move toward the closed position A 1  illustrated in  FIG. 9A . At this time, the front cover  3  presses up the conveying unit  73  at the open position B 2  in process of the closing operation. 
     Namely, as illustrated in  FIG. 10 , a contacting part  294  at the side of the front cover  3  contacts a contacting part  295  at the side of the conveying unit  73  to press up it so as to lift up from the lower side, which makes the conveying unit  73  at the open position B 2  move toward the closed position B 1 . Then, as the front cover  3  approaches the closed position A 1  and the conveying unit  73  approaches the closed position B 1 , in place of the contacting parts  294  and  295  at the side of the front cover  3  and the side of the conveying unit  73  which have come into contact with each other so far, as illustrated in  FIG. 14A , the cam follower portion  284  of the shaft member  282  at the side of the front cover  3  and the cam portion  285  at the entrance of the catching concave portion  287  at the side of the conveying unit  73  come into contact with each other. 
     In this state, when the front cover  3  further moves toward the closed position A 1 , the shaft member  282  moves such that the cam follower portion  284  is guided by the cam portion  285 , and the cam follower portion  284  and the notch  286  pass though the opening portion  288 . When the cam follower portion  284  passes through the convex portion  291  on the boarder between the cam portion  285  and the catching concave portion  287 , as illustrated in  FIG. 14B , the cam follower portion  284  is inclined centering on the convex portion  291 , and moment in a direction of an arrow R 282  is applied to the shaft member  282 . Due to this moment, moment in a direction of an arrow R 283 , i.e., a direction of engaging with respect to the engaging member  281  is applied to the hook member  283  integrated with the shaft member  282 . Then, as illustrated in  FIG. 14C , at substantially the same time when the shaft member  282  is brought to make an engagement with the catching concave portion  287 , the catching portion  283   a  of the hook member  283  is brought to make an engagement with the engaging member  281 . 
     The shaft member  282  brought to make an engagement with the catching concave portion  287  as described above is in an engagement in a state of being rotated slightly from an angle at which the cam follower portion  284  and the notch  286  can pass through the opening portion  288 . Therefore, unless directions of the cam follower portion  284  and the notch  286 , and a direction of the opening portion  288  are made to match to one another by rotating the shaft member  282  from the outside, there are no cases in which the shaft member  282  drops out of the catching concave portion  287 . Namely, there are no cases in which the engaging state (lock state) of the hook member  283  with respect to the engaging member  281  is released. 
     In accordance with the above operations, the front cover  3  is positioned at the closed position A 1  with respect to the apparatus main body  2 , and further, the conveying unit  73  is positioned at the closed position B 1 . Note that, in order to release the engaging state (lock state) of the hook member  283  with respect to the engaging member  281 , the buttons  93  of the release levers  80  described in the first embodiment should be pulled toward the front side. Thereby, the hook member  283  is rotated in a releasing direction opposite to the engaging direction via the connecting member  88  (refer to  FIG. 8  and the like) to which the hook member  283  is connected, and the engagement of the hook member  283  is released, which makes it possible to open the front cover  3 . The hook member  283  whose engagement is released is held at a position to release the engagement illustrated in  FIG. 14A  by a restricting member (not shown) such as a stopper. 
     In accordance with the image forming apparatus  1 A according to the second embodiment described above, because the hook member  283  to position the front cover  3  with respect to the apparatus main body  2  and the shaft member  282  to position the conveying unit  73  with respect to the apparatus main body  2  are integrally structured, it is possible to improve an accuracy of positioning of the front cover  3  and the conveying unit  73  with respect to the apparatus main body  2 . Thereby, it is possible to appropriately set nip pressures at the pair of registration rollers  57 , the driving roller  43  and the secondary transfer roller  45 , and the pairs of reconveying rollers  70  and  71  illustrated in  FIG. 9A , which makes it possible to smoothly convey a sheet. 
     Further, the engaging member  281  at the side of the image forming apparatus main body  2  serving as a reference for positioning of the front cover  3 , and the shaft member  282  serving as a reference for positioning of the conveying unit  73  are located at substantially a same height at the closed position A 1  of the front cover  3  and the closed position B 1  of the conveying unit  73 . Or, the engaging member  281  at the side of the apparatus main body  2  is substantially located on a circumference with a radial of a length from the pivot point  85  of the front cover  3  serving as a reference up to the center of the shaft member  282 . It is possible to improve an accuracy of positioning thereby as well. 
     Moreover, the cam follower portion  284  is provided to the shaft member  282 , and the cam portion  285  is provided to the conveying unit  73 . Therefore, shortly before the front cover  3  is moved from the open position A 2  toward the closed position A 1  to bring the shaft member  282  into making an engagement with the catching concave portion  287 , moment to rotate the hook member  283  toward the engaging direction is applied to the shaft member  282  by the cam follower portion  284  and the cam portion  285 , and the catching portion  283   a  of the cam member  283  is brought to make an engagement with the engaging member  281 . Accordingly, a biasing member such as an extension spring to bias the hook member  283  to the engaging direction is omitted positively, and it is possible to simplify the structure by that much. 
     In the second embodiment, the example that the cam follower portion  284  is provided to the side of the shaft member  282 , and the cam portion  285  is provided to the side of the conveying unit  73  has been described. In contrast thereto, a cam portion may be provided to the side of the shaft member  282 , and a cam follower portion may be provided to the side of the conveying unit  73 . Namely, provided that a cam follower portion disposed in one side and a cam portion disposed in the other side between the front cover  3  and the conveying unit  73  come into contact with each other by an operation of closing the front cover  3 , and the both relatively move, which makes it possible to generate moment to rotate the hook member  283  to the engaging direction with respect to the shaft member  282 , any structure can be employed in which a cam follower portion and a cam portion are disposed in any one of the front cover  3  and the conveying unit  73 . 
     Third Embodiment 
     In a third embodiment, an embodiment in which an opening and closing aid mechanism  380  to bias the front cover  3  toward a direction of a closed position is provided, will be described. The opening and closing aid mechanism  380  is installed between the apparatus main body  2  and the front cover  3 . Hereinafter, the opening and closing aid mechanism  380  will be described with reference to  FIGS. 15 to 17 . Thereamong,  FIG. 15  is a left side view illustrating a state of the opening and closing aid mechanism  380  when the front cover  3  is disposed at the closed position A 1  (refer to  FIG. 9A ).  FIG. 16  is a perspective view illustrating a state of the opening and closing aid mechanism  380  when the front cover  3  is disposed at a substantially intermediate position between the closed position A 1  and the open position A 2  (refer to  FIG. 9B ).  FIG. 17  is a left side view illustrating a state of the opening and closing aid mechanism  380  when the front cover  3  is disposed at the open position A 2 . 
     The opening and closing aid mechanism  380  has a guide member  381  fixed to the front cover  3 , a holding link  382  having a pivot point  382   a  at the side of the apparatus main body  2 , and an extension spring  383  connecting the holding link  382  and the front cover  3 . 
     The guide member  381  is, as illustrated in  FIG. 15 , fixed to a portion slightly lower than a substantially intermediate position in a vertical direction on a side surface  3   b  of the front cover  3 , and has a guide pin (not shown) moving so as to follow a guide groove  382   b  of the holding link  382 , and a rectangular retaining portion  381   a  fixed to a head portion of the guide pin to prevent the holding link  382  from dropping off. Note that the side surface  3   b  of the front cover  3  is to be exposed by detaching the left panel  14  from the front cover  3  illustrated in  FIG. 4 . 
     The holding link  382  is formed of a laminar long member, and the pivot point  382   a  of the base end portion is supported pivotally by an attaching portion  2   b  at the side of the apparatus main body  2 . The attaching portion  2   b  is located at the lower end side of an anterior surface  2   a  of the apparatus main body  2 . Further, a position of the attaching portion  2   b  corresponds to an upper side of the anterior surface of the paper feed cassette  25  in a state of being housed in the apparatus main body  2 , and the pivot point  85  of the front cover  3  is located below the attaching portion  2   b.    
     The guide groove  382   b  is formed in the leading end side (the upper end side in  FIG. 15 ) of the holding link  382 . The guide groove  382   b  is formed from a substantially intermediate position in the longitudinal direction up to the leading end side of the holding link  382 , and a groove width thereof is set to be slightly wider than a diameter of the guide pin of the guide member  381  at the side of the front cover  3 , and narrower than a length at the long side of the retaining portion  381   a . The guide pin is capable of relatively moving along the guide groove  382   b , and when the front cover  3  is disposed at the closed position A 1 , the guide pin is located at an end portion  382   c  at the base end side of the guide groove  382   b , and when the front cover  3  is disposed at the open position A 2  as illustrated in  FIG. 17 , the guide pin is located at an end portion  382   d  at the leading end side. A connecting portion  382   e  is provided in the vicinity of the end portion at the leading end side of the guide groove  382   b  in the holding link  382 . Other end portion  383   b  of the extension spring  383  which will be described later is connected to the connecting portion  382   e.    
     The extension spring  383  connects the holding link  382  and the front cover  3 . With respect to the extension spring  383 , a one end portion  383   a  located at the upper end side in  FIG. 15  is connected to a connecting portion  3   d  at the upper end side of the side surface  3   b  of the front cover  3 , and the other end portion  383   b  is connected to a connecting portion  382   e  at the side of the holding link  382 . Spring force of the extension spring  383  is set to an extension that, when the front cover  3  is disposed at the open position A 2  illustrated in  FIG. 17 , the extension spring  383  becomes a maximum length, and even at that time, it is possible to bias the front cover  3  toward the closing direction, but not enough to move the front cover  3  from the open position A 2 . 
     In the present embodiment, a target to which the other end portion  383   b  of the extension spring  383  is connected is set to, not the side of the apparatus main body  2 , but the holding link  382  supported pivotally by the apparatus main body  2 . Therefore, it is possible to restrict a moving range of the extension spring  383  according to opening and closing of the front cover  3  within an area occupied by the side surface  3   b  at a time of facing to view the side surface  3   b  of the front cover  3 . Thereby, for example, when the front cover  3  is disposed at the open position A 2  in order to remove a jammed paper generated in the sheet conveying path  52  or the sheet reconveying path  53  (refer to  FIG. 3 ), there are no cases in which the extension spring  383  gets in the way of a work for removing the jammed paper. 
     With respect to the opening and closing aid mechanism  80 , as illustrated in  FIG. 15 , when front cover  3  is disposed at the closed position A 1 , the holding link  382  takes a substantially standup position so as to locate its leading end side slightly forward, and the extension spring  383  as well is provided to stretch in substantially the same direction as the holding link  382 . The guide member  381  integrated with the front cover  3  is located at the end portion  382   c  at the base end side of the guide groove  382   b . At this time, given that a straight line connecting the pivot point  382   a  of the holding link  382  and the connecting portion  3   d  at the side of the front cover  3  is a virtual straight line S, the guide member  381  integrated with the front cover  3  and the connecting portion  382   e  at the side of the holding link  382  are substantially on the virtual straight line S. 
     Further, the pivot point  85  of the front cover  3  is to be an extension of the virtual straight line S to be extended downward. In other words, in order of the lower side, the pivot point  85  of the front cover  3 , the pivot point  382   a  of the holding link  382 , the guide member  381 , the connecting portion  382   e  of the holding link  382 , and the connecting portion  3   d  of the front cover  3  are substantially on the virtual straight line S. Accordingly, in this state, the extension spring  383  becomes a minimum length (extends at minimum), which does not generate force to bias the front cover  3  toward the closing direction. Therefore, when the operator starts opening the front cover  3 , the extension spring  383  does not get in no way. 
     Here, when paper jam is caused in the sheet conveying path  52  or the sheet reconveying path  53 , the operator opens the front cover  3  in order to remove the jammed paper. It is possible for the operator to start opening the front cover  3  by releasing the locking by operating the buttons  93  of the release levers  80  described above in the first embodiment. Thereby, as illustrated in  FIG. 15 , the upper end side of the front cover  3  is swung in a direction of an arrow R 3   a  centering on the pivot point  85  at the lower end side. According to this, the guide member  381  integrated with the front cover  3  rotates in the same direction, and the holding link  382  is rotated so as to fall in a direction of an arrow R 382  centering on the pivot point  382   a  by the guide member  381 . 
     Then, the front cover  3  passes through a state of being half opened illustrated in  FIG. 16 , to change its position to a state of being completely opened illustrated in  FIG. 17 , i.e., the open position A 2 . In  FIG. 17 , a trajectory S 1  of the connecting portion  3   d  at the side of the front cover  3  and a trajectory S 2  of the connecting portion  382   e  at the side of the holding link  382  during the movement of front cover  3  from the closed position A 1  up to the open position A 2  are illustrated. As shown in the drawing, the pivot point  382   a  at the side of the holding link  382  is located superior to the pivot point  85  at the side of the front cover  3 , and a distance from the pivot point  382   a  up to the connecting portion  382   e  of the holding link  382  is shorter than a distance from the pivot point  85  to the connecting portion  3   d  of the front cover  3 . 
     Therefore, given that an angle at which a straight line connecting the pivot point  382   a  and the connecting portion  382   e  is against the virtual straight line S connecting the pivot point  382   a  at the side of the holding link  382  and the connecting portion  3   d  at the side of the front cover  3 , is an inclined angle α of the holding link  382  against the front cover  3 , the relationship is made such that the inclined angle α is 0 degree when the front cover  3  is disposed at the closed position A 1 , and becomes a maximum when the front cover  3  is disposed at the open position A 2 , and between the closed position A 1  and the open position A 2 , the greater the open angle of the front cover  3  is, the greater the inclined angle α becomes. In the same way, the greater the open angle of the front cover  3  is, the greater the distance between the connecting portion  3   d  at the side of the front cover  3  and the connecting portion  382   e  at the side of the holding link  382  becomes, which makes extension of the extension spring  383  greater. 
     When the front cover  3  is disposed at the closed position A 1 , the inclined angle α is 0 degree, and a length (extension) of the extension spring  383  becomes a minimum. The greater the open angle of the front cover  3  is, the greater the inclined angle α becomes, which makes a length (extension) of the extension spring  383  greater. Then, when the front cover  3  is disposed at the open position A 2 , both of the inclined angle α the length (extension) of the extension spring  383  are maximized. When the extension spring  383  pulls the holding link  382  at the inclined angle α, it is possible to bias the guide member  381  toward the closing direction of the front cover  3  via the holding link  382 . This biasing force is, in principle, generated when the holding link  382  is at the inclined angle α, i.e., when the front cover  3  is opened even slightly from the closed position A 1 . 
     In this way, in the present embodiment, it is possible to apply biasing force in the closing direction to the front cover  3  over the entire moving range of the front cover  3 , i.e., the entire region from the closed position A 1  to the open position A 2 , even with a simple structure. Moreover, when an open angle of the front cover  3  is made greater, the greater the own weight of the front cover  3  is loaded on the operator, the greater the biasing force becomes. 
     Accordingly, because load to be supported by his or her own hands during an opening and closing operation of the front cover  3  is reduced, it is possible for the operator to lightly carry out opening and closing of the front cover  3 . Further, for example, even when the operator unlinks his or her own hands from the front cover  3  during an opening and closing operation of the front cover  3 , because the front cover  3  is biased in the closing direction, even in a case in which the front cover  3  is opened to the open position A 2  against the biasing force, it is possible to cushion the shock, and prevent the holding link  3 , the guide member  381 , and the like from being damaged. Accordingly, provided that part of the sheet conveying paths is built into the front cover  3 , it is useful especially for a case in which its own weight of the front cover  3  is heavy. 
     Further, the guide member  381  and the holding link  382  structuring the opening and closing aid mechanism  380  function as stoppers to retain the front cover  3  at the open position A 2 . Accordingly, with respect to the image forming apparatus which has already a same shaped holding link, it is possible to construct the opening and closing aid mechanism  380  by merely adding the above-described extension spring  383 . 
     In the above-described third embodiment, the opening and closing aid mechanism  380  has been described by using a case in which the opening and closing aid mechanism  380  is disposed at the left end side of the front cover  3 , as an example. Instead of that, the opening and closing aid mechanism  380  may be disposed at the right end side of the front cover  3 , or both of the left end side and the right end side of the front cover  3 . 
     Note that the invention having the following structures is mainly included in the above-described specific embodiments. 
     An image forming apparatus according to an aspect of the present invention comprising: an image forming unit which forms an image on a sheet; a sheet conveying section which conveys the sheet on which the image is formed; an apparatus main body in which the image forming unit and the sheet conveying section are installed; an operating unit to input operational information; a front cover to which the operating unit is mounted to be supported in a freely openable/closable manner by the apparatus main body, the front cover is capable of changing in position between a closed position to cover an anterior surface side of the apparatus main body and an open position to expose a part of the apparatus main body; and a retention mechanism which includes a lock member and a release member, the retention mechanism retains the front cover at the closed position with respect to the apparatus main body, wherein the lock member is a member which is supported by one of the apparatus main body and the front cover so as to be capable of making an engagement with and being detached from the other one, and the release member is a member to release engagement of the lock member, and the release member is installed at a region located posterior to the operating unit and a region facing a back surface side as viewed from an opening and closing direction of the front cover. 
     In this case, it is preferable that the front cover is supported at its lower end side in a freely openable/closable manner by the apparatus main body, and is opened such that its upper end side is separated away from the apparatus main body at the open position, the operating unit is disposed at the upper end side of the front cover, and the release member is installed at a side posterior to the operating unit at the upper end side of the front cover. 
     According to such an image forming apparatus, it is difficult to view the release member from a front surface side (the opening and closing direction of the front cover) of the image forming apparatus, where an operator is located at the time of operating the operating unit, and it is possible to hard to carry out an incorrect operation. 
     In the above-described structure, it is preferable that the release member is disposed within an operating range in which it is possible for an operator operating the operating unit at a front surface side opposed to the front cover of the apparatus main body to operate the release member. According to this structure, when the operator makes an attempt to operate the release member positively with the intention of operating it, it is possible for the operator to easily operate it without moving especially from the front surface side. 
     In the above-described structure, it is preferable that a direction of operating the release member by the operator at the time of releasing the engagement of the lock member and the opening and closing direction of the front cover are substantially the same. According to this structure, it is possible for the operator to smoothly carry out an operation of the release member to an operation of opening the front cover as a series of operations along a same direction. 
     In the above-described structure, it is preferable that the sheet conveying section has a sheet conveying path which is disposed at the anterior surface side of the apparatus main body, to convey the sheet therethrough, the front cover has a guide portion structuring a part of the sheet conveying path when the front cover is disposed at the closed position, and when the front cover is moved to the open position, the sheet conveying path is opened. According to this structure, when the front cover is opened, it is possible to easily open the part of the sheet conveying path according to this without any special opening operation. 
     In the above-described structure, it is preferable that a paper discharge tray which is provided at a superior surface of the apparatus main body, and to which a sheet after image formation is discharged, and a sheet discharge opening which is provided at a position at which an upper end side back surface of the front cover exists, and from which a sheet after image formation is discharged toward the paper discharge tray, are further included, and the release member is disposed at a peripheral portion of the sheet discharge opening. According to this structure, it is possible to make the release member hard to be viewed from the front surface side, which is further hard to carry out an incorrect operation. 
     In this case, it is more preferable that the release members are disposed at a left side and a right side of the sheet discharge opening, which makes it possible to release the lock member by operating one of the release members. According to this structure, it is possible for the operator to release the lock member by operating at least one, which is easier to operate, of the release members. 
     In the above-described structure, it is preferable that the lock member is a lock member assembly supported by the front cover, the lock member assembly includes the following; a slide member which is a rod-like member having a first end portion and a second end portion, the slide member is capable of sliding by applying pressing force to the first end portion from the release member; a rotary member with which the second end portion of the slide member is brought into contact, the rotary member rotates by the sliding; a hook member which makes an engagement with and is detached from a predetermined engaging member provided to the apparatus main body; and a connecting member in a belt shape having a first connecting portion to be connected to the rotary member and a second connecting portion to be connected to the hook member. According to this structure, it is possible to install the front cover in a mode in which the lock member is simple and does not take up much space. 
     In the above-described structure, it is preferable that the sheet conveying section has a sheet conveying path which is disposed at the anterior surface side of the apparatus main body, to convey the sheet therethrough, and image forming apparatus further includes a conveying unit whose lower end side is supported swingablly by the apparatus main body, the conveying unit is capable of changing in position between a closed position to structure a part of the sheet conveying path by being interposed between the front cover disposed at the closed position and the anterior surface side of the apparatus main body, and an open position to open the sheet conveying path according to opening of the front cover. 
     In particular, it is preferable that the sheet conveying section has a sheet conveying path which is disposed at the anterior surface side of the apparatus main body, to convey the sheet therethrough, and image forming apparatus further includes a conveying unit whose lower end side is supported pivotally by the apparatus main body, the conveying unit is capable of changing in position between a closed position to structure a part of the sheet conveying path by being interposed between the front cover disposed at the closed position and the anterior surface side of the apparatus main body, and an open position to open the sheet conveying path according to opening of the front cover, and an engaging member provided to the apparatus main body, and the lock member is a lock member assembly including the following; a shaft member which is installed along a horizontal direction at a side of the front cover, and the shaft member presses to position the conveying unit at the closed position when the front cover is moved from the open position to the closed position; and a hook member which is structured integrally with the shaft member, and is capable of making an engagement with and being detached from the engaging member, the hook member positions the front cover at the closed position at a time of engaging. 
     According to this structure, the front cover is positioned at the closed position with respect to the apparatus main body by making the hook member of the lock member assembly engage with the engaging portion at the side of the apparatus main body. Further, the conveying unit is positioned at the closed position with respect to the apparatus main body by the shaft member integrated with the hook member of the lock member assembly. Namely, because the front cover and the conveying unit are respectively positioned at the closed position and the closed position with respect to the apparatus main body by the hook member and the shaft member integrated with one another, it is possible to position the front cover and the conveying unit with respect to the apparatus main body with high accuracy. 
     In the above-described structure, it is preferable that the conveying unit has a catching concave portion to position the conveying unit at the closed position by making an engagement with the shaft member in a state in which the front cover is disposed at the closed position. According to this structure, it is possible to secure relative positioning accuracy of the front cover and the conveying unit with high accuracy by making an engagement between the shaft member and the catching concave portion. 
     In the above-described structure, it is preferable that the hook members are respectively installed at both ends of the shaft member, and the engaging members are installed at positions corresponding to the respective hook members at the side of the apparatus main body. According to this structure, it is possible to prevent the front cover and the conveying unit from being disposed at the closed position and the closed position in a state in which the left end sides and the right end sides are shifted. 
     In the above-described structure, it is preferable that a cam portion formed at one of an entrance of the catching concave portion at the side of the conveying unit and the shaft member, and a cam follower portion formed at the other one are included, the cam portion and the cam follower portion are configured to rotate the hook member toward an engaging direction via the shaft member due to the cam portion and the cam follower portion contacting to have relative movement when the front cover is moved from the open position to the closed position. According to this structure, it is possible to smoothly make the hook member make an engagement with the engaging member without providing a biasing member such as a spring. 
     In the above-described structure, it is preferable that the sheet conveying section has a first sheet conveying path structured between the conveying unit disposed at the closed position and the anterior surface side of the apparatus main body, and a second sheet conveying path, which is formed between the conveying unit disposed at the closed position and the front cover, to guide a sheet after image formation again to the image forming unit. 
     According to this structure, it is possible to secure high positioning accuracy of the first and second sheet conveying paths. Therefore, for example, in a case in which pairs of conveying rollers are disposed at a sheet conveying path and a sheet reconveying path, and the pairs of conveying rollers are composed pairs of rollers which are separated away from each other in an open state of the front cover and an open state of the conveying unit, it is possible to set nip pressures of conveying nips of the pairs of conveying rollers when the front cover is disposed at the closed position, and the conveying unit is disposed at the closed position, to be appropriate levels, which makes it possible to smoothly convey a sheet. 
     In the above-described structure, it is preferable that an opening and closing aid mechanism which is installed between the apparatus main body and the front cover, to aid an opening and closing operation of the front cover, the opening and closing aid mechanism includes the following; a guide member provided to project to a side surface of the front cover; a holding link whose base end portion is supported pivotally by the apparatus main body, and which has a guide groove in a long hole shape to guide a movement of the guide member at a side of a leading end, the holding link holds the front cover at the open position by bringing the guide member into contact with the leading end of the guide groove; and an extension spring whose one end portion is connected to the side surface of the front cover, and whose other end portion is connected to the holding link, the extension spring biases the front cover toward a closing direction via the guide member by biasing the holding link. 
     According to this structure, with a simple structure in which the holding link supported pivotally by the apparatus main body is biased by the extension spring, it is possible to effectively bias the front cover toward the closing direction over the entire area of a moving range of the front cover. Further, for example, because it is possible to dispose the holding link in the vicinity of the base end portion (lower end side) of the front cover, it is possible to make a structure hard to be in the way at the time of opening the front cover as compared with a structure in which the apparatus main body and the front cover are connected with an extension spring. 
     In the above-described structure, it is preferable that, with respect to the extension spring, the one end portion is connected to the upper end side of the front cover, and the other end portion is connected to the leading end side of the holding link. According to this structure, because the other end portion of the extension spring is connected to a position distant from the base end portion serving as a center of swinging of the holding link, as compared with a case in which the extension spring is connected to a position near the base end portion, it is possible to bias the cover member toward the closing direction by great force. 
     In the above-described structure, it is preferable that an operating range of the extension spring is limited within an area occupied by the side surface at a time of facing to view the side surface of the front cover. According to this structure, there is no risk that the extension spring is in the way when the cover member is opened. 
     An image forming apparatus according to another aspect of the present invention, comprising: an image forming unit which forms an image on a sheet; a sheet conveying section which conveys the sheet on which the image is formed; an apparatus main body in which the image forming unit and the sheet conveying section are installed; a front cover which is supported in a freely openable/closable manner by the apparatus main body, the front cover is capable of changing in position between a closed position to cover an anterior surface side of the apparatus main body and an open position to expose a part of the apparatus main body; and a retention mechanism which includes a lock member and a release member, the retention mechanism retains the front cover at the closed position with respect to the apparatus main body, wherein the front cover has front face panels located at outer side surfaces, and the front face panels includes an anterior panel and an upper panel continuing to an upper end of the anterior panel, the lock member is a member which is supported by one of the apparatus main body and the front cover so as to be capable of making an engagement with and being detached from the other one, and the release member is a member to release engagement of the lock member, and the release member is installed at a back surface side of the upper panel as viewed from an opening and closing direction of the front cover. 
     This application is based on patent application Nos. 2007-105666, 2007-134915 and 2007-155930 filed in Japan, the contents of which are hereby incorporated by references. 
     As this invention may be embodied in several forms without departing from the spirit of essential characteristics thereof, the present embodiment is therefore illustrative and not restrictive, since the scope of the invention is defined by the appended claims rather than by the description preceding them, and all changes that fall within metes and bounds of the claims, or equivalence of such metes and bounds are therefore intended to be embraced by the claims.