Patent Publication Number: US-6700863-B2

Title: Disk cartridge

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a disk cartridge having a cartridge housing in which a disk-like recording medium such as an optical disk, a magneto-optical disk and a magnetic disk is stored in a disk compartment so as to become freely rotatable, and particularly to a disk cartridge in which a pair of shutter members are opened and closed to open and close an opening portion by rotating a middle shell which forms a disk compartment between it and an upper shell or a lower shell and part of a disk-like recording medium is exposed from the opening portion when the opening portion is opened. 
     2. Description of the Related Art 
     As a disk cartridge in which a disk-like recording medium in and/or from which audio information, video information or information such as computer data can be recorded and/or reproduced is rotatably stored in a cartridge housing, there is known a disk cartridge having an arrangement shown in FIG. 1, for example. This disk cartridge  1  is a disk cartridge incorporating therein a write once magneto-optical disk  4  in which a user can record and write information such as computer data later on. This disk cartridge has an outward shape shown in FIG.  1 . 
     As shown in FIG. 1, this disk cartridge  1  includes a cartridge housing  2  comprised of a pair of an upper shell  2   a  and a lower shell  2   b , a magneto-optical disk  4  rotatably stored within a disk compartment  3  of this cartridge housing, and the like. The cartridge housing  2  has on its upper and lower surfaces defined an upper and lower opening portion  5  which is extended from the central portion to one side. This opening portion  5  can be opened and closed by a shutter member  6  which can be slid along one side. Reference numeral  6   a  designates a presser member which can prevent the tip end portion of the shutter member  6  from being extracted. 
     A disk-like metal center hub  7  is provided at the central portion of the magneto-optical disk  4 . This center hub  7  is disposed at the position opposing to the inner end portion of the opening portion  5 , i.e., at substantially the central portion of the cartridge housing  2 . A turntable provided on an information recording and reproducing apparatus body side is mounted on this center hub  7  and the magneto-optical disk is fixed to the center hub by chucking the turntable, whereby the magneto-optical disk  4  is rotated at a predetermined velocity (e.g., constant linear velocity). In this case, an information signal is recorded on or reproduced from the magneto-optical disk  4  by an action of a magneto-optical pickup device whose head portion is inserted into the opening portion  5 . 
     However, in the related-art disk cartridge  1  having the above arrangement, the shutter member  6  which opens and closes the opening portion  5  has a U-like shaped cross-section. This shutter member  6  is fitted into one side of the cartridge housing  2  and the shutter member  6  is slid along one side of the cartridge housing  2  to thereby open and close the opening portion  5 . For this reason, a disk recording and reproducing apparatus side requires a shutter opening and closing mechanism for sliding the shutter member  6  to open and close the opening portion  5 , and the related-art shutter opening and closing mechanism is relatively complicated in structure. Further, it is unavoidable that a large space is produced between the cartridge housing  2  and the shutter member  6 , and therefore the disk cartridge  1  cannot be prevented from being smudged by very small dusts and the like. 
     In recent years, in the stream in which an optical disk is increasing its storage capacity/increasing its recording density, a recording pattern is becoming narrower in pitch and is increasing a linear density. When a recording surface of an optical disk or a magneto-optical disk is smudged by dusts, read or write beam is shielded or when the information recording surface of the optical disk is damaged by scratches, information cannot be read or written normally. For this reason, in the related-art disk cartridge  1 , as shown in FIG. 1, the shutter member  6  having substantially the U-like shaped cross-section is attached to one side of the cartridge housing  2  so as to become slidable and is slid along one side of the cartridge housing  2  to thereby open and close the opening portion  5  so that the information recording surface of the optical disk or the like can be protected. 
     However, as the optical disk is further increasing a storage capacity/increasing a recording density, very small dusts which are not so influential in the related art becomes more influential in reading and writing of data. Therefore, the shutter member having the above U-like shape cannot prevent the disk cartridge  1  from being smudged by very small dusts. In this case, when the information recording surface of the magneto-optical disk  4  is smudged by very small dusts entered into the cartridge housing  2 , the information recording surface of the magneto-optical disk is damaged by dusts so that information cannot be read out from and written in the magneto-optical disk normally. 
     In the above disk cartridge  1  according to the related art, the shutter member  6  is located on the disk cartridge  1  at its position in which users can easily touch the shutter member  6 . There is then the risk that a user will slide the shutter member  6  to open the opening portion  5  by mistake. In addition, since the shutter member  6  is linearly slid along one side of the cartridge housing  2 , there arises a problem that the surface area of the cartridge housing  2  cannot be reduced. 
     SUMMARY OF THE INVENTION 
     In view of the aforesaid aspect, it is an object of the present invention to provide a disk cartridge in which an opening portion in and from which a turntable, an optical head and the like are inserted and ejected is widened as much as possible so that two optical heads can be used simultaneously. 
     It is another object of the present invention to provide a disk cartridge in which an optical head can approach the innermost periphery of a disk-like recording medium with high efficiency. 
     It is a further object of the present invention to provide a disk cartridge in which a middle shell is made rotatable, a pair of shutter members are opened and closed based on the rotation of this middle shell and end faces of the pair of shutter members are contacted with each other so that the pair of shutter members can be opened and closed smoothly and reliably. 
     It is yet a further object of the present invention to provide a disk cartridge in which a space between the pair of shutter members is reduced as much as possible by pressing the pair of shutter members against each other so that a dustproof property and an airtightness of the large opening portion can be improved. 
     010 According to an aspect of the present invention, there is provided a disk cartridge which is comprised of a cartridge housing in which a disk compartment is formed by overlapping a pair of shells, a disk-like recording medium rotatably stored within the disk compartment and a pair of shutter members rotatably attached to the cartridge housing and exposing part of the disk-like recording medium by opening and closing an opening portion provided on the cartridge housing when they are moved, wherein the opening portion is extended in the diametrical direction of the disk-like recording medium through the central portion of the lower shell or the upper shell. 
     According to another aspect of the present invention, in the disk cartridge, the cartridge housing includes a middle shell rotatably supported by the upper shell and the lower shell and the opening portion is formed through the middle shell and the lower shell or the upper shell. 
     According to still another aspect of the present invention, in the disk cartridge, the pair of shutter members are comprised of a combination of a pair of substantially semicircular same plate materials, the pair of shutter members are symmetrically disposed on one surface of the middle shell across the opening portion, the guide groove is provided on one side portion of a chord side in each of the shutter members and a shaft portion provided on the other side portion of the chord side is rotatably supported to the middle shell. 
     According to a further aspect of the present invention, in the disk cartridge, a pair of guide grooves are provided on one of the pair of shutter members and the cartridge housing, a pair of operation convex portions which are slidably engaged with the guide grooves are provided on the other of the pair of shutter members and the cartridge housing, the guide grooves are moved along the operation convex portions based on the rotation of the middle shell, whereby the pair of shutter members are enabled to open and close the opening portion. 
     In accordance with a further aspect of the present invention, there is provided a disk cartridge which is comprised of a cartridge housing in which a disk compartment is formed within an upper shell and a middle shell or the middle shell and a lower shell by overlapping the upper shell, the middle shell and the lower shell and in which the middle shell is rotatably supported by the upper shell and the lower shell, a disk-like recording medium is stored within the disk compartment so as to become freely rotatable, a pair of shutter members are attached to the middle shell in such a manner that they can be moved in the flat surface direction on the same plane and a shutter opening and closing mechanism for tightly closing the opening portion provided on the cartridge housing and from which part of the disk-like recording medium is exposed by moving the pair of shutter members based on the rotation of the middle shell such that end faces of the pair of shutter members are pressed against each other. 
     In accordance with yet a further aspect of the present invention, in the disk cartridge, the shutter opening and closing mechanism includes a pair of guide grooves provided on one of the pair of shutter members and the cartridge housing, a pair of operational convex portions provided on the other of the pair of shutter members and the cartridge housing and which are slidably engaged with the guide grooves and the pair of shutter members are enabled to open and close the open and close portion by moving the guide grooves along the operational convex portions based on the rotation of the middle shell. 
     In accordance with yet a further aspect of the present invention, in the disk cartridge, the pair of guide grooves include resilient members for imparting spring force to the pair of operational convexs such that the pair of shutter members are spring-biased in the flat surface direction when the pair of shutter members are closed. 
     In accordance with still a further aspect of the present invention, in the disk cartridge, the pair of shutter members are comprised of a combination of a pair of substantially semicircular same plate-like materials, the pair of shutter members are symmetrically disposed on one surface side of the middle shell across the opening portion, the guide grooves are provided on one side portions of chord sides of the respective shutter members and pivots provided on the other side portions of the chord sides are supported to the middle shell so as to become freely rotatable. 
     14 With the above arrangement, in the disk cartridge including the cartridge housing having the disk compartment, the disk-like recording medium and the pair of shutter members, since the opening portion is extended in the diametrical direction of the disk-like recording medium through the central portion of the cartridge housing, two recording and/or reproducing head portions can simultaneously be inserted into the opening portion so that two recording and/or reproducing operations can be executed at the same time. Furthermore, the two recording and/or reproducing head portions can approach the innermost periphery of the disk-like recording medium and therefore, the disk cartridge can approach the innermost periphery of the disk-like recording medium with high efficiency. 
     According to the disk cartridge of the present invention, since the cartridge housing includes the middle shell in addition to the upper shell and the lower shell, the middle shell is rotatably supported by the upper shell and the lower shell and the opening portion is provided through the middle shell and the upper shell or the lower shell, there can be formed the opening portion into and from which two recording and/or reproducing head portions can be inserted and ejected simultaneously, and the opening portion can be opened and closed by the pair of shutter members. 
     16 According to the disk cartridge of the present invention, since the pair of shutter members are comprised of the combination of two substantially semicircular same plate materials, the pair of shutter members are symmetrically disposed across the opening portion, the guide grooves are provided on one side portions of the respective shutter members, the pivots are provided on the other side portions and the pivots are supported to the middle shell so as to become freely rotatable, the opening portion of the wide range can be opened and closed while the areas of the shutter members can be reduced. Therefore, the efficiency with which the opening portion is opened and closed can be increased, and the whole of the disk cartridge can be made compact and thin. 
     Further, according to the disk cartridge of the present invention, since the pair of guide grooves are provided on one of the pair of shutter members and the cartridge housing, the pair of operational convexs are provided on the other of the pair of guide grooves and the guide grooves are moved along the operational convexs based on the rotation of the middle shell, the pair of shutter members can be moved with high reliability so that the opening portion can be opened and closed smoothly and reliably. 
     Further, according to the disk cartridge of the present invention, in the disk cartridge comprising the cartridge housing having the disk compartment, the disk-like recording medium and the pair of shutter members, since the disk cartridge includes the shutter opening and closing mechanism for moving the pair of shutter members based on the rotation of the middle shell so that the end faces of the pair of shutter members are pressed against each other, the pair of shutter members can be contacted with each other reliably and strongly by rotating the middle shell. Therefore, the disk compartment can be prevented from being smudged by very small dusts and the like by reducing the space between the pair of shutter members and the cartridge housing. Hence, the dustproof property of the cartridge housing can be improved. Moreover, the disk cartridge can be reduced in thickness and improved in space factor so that the whole of the disk cartridge can be made compact and thin. 
     Further, according to the disk cartridge of the present invention, since the pair of operational convexs are moved along the pair of guide grooves based on the rotation of the middle shell, the pair of shutter members can be rotated smoothly and reliably. 
     Further, according to the disk cartridge of the present invention, since the shutter open and close mechanism presses the pair of shutter members, the two shutter members can be closely contacted with each other so that the dustproof property of the cartridge housing can be improved. 
     Furthermore, according to the disk cartridge of the present invention, since the pair of shutter members are comprised of a combination of two substantially semicircular same plate materials and the pair of shutter members are symmetrically disposed across the opening portion, the pair of shutter members can be operated with high reliability. Therefore, the opening portion can be opened and closed smoothly and reliably, and the whole of the disk cartridge can be made compact and thin. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view showing a disk cartridge according to the related art; 
     FIG. 2 is an exploded perspective view showing a disk cartridge according to a first embodiment of the present invention and illustrates the same disk cartridge from the lower surface side; 
     FIG. 3 is a perspective view showing the disk cartridge shown in FIG. 2 from the lower surface side and illustrating the state in which an opening portion is closed by closing a shutter mechanism; 
     FIG. 4 is a perspective view showing the disk cartridge shown in FIG. 2 from the lower surface side and illustrates the state in which the opening portion is opened by opening the shutter mechanism. 
     FIG. 5 is a plan view showing a lower surface of an upper shell of the disk cartridge shown in FIG. 2; 
     FIG. 6 is a plan view showing a lower surface of a middle shell of the disk cartridge shown in FIG. 2; 
     FIG. 7 is a plan view showing an upper surface of a lower shell of the disk cartridge shown in FIG. 2; 
     FIG. 8 is a perspective view showing the state in which an opening portion is opened by opening a pair of shutter members attached to the middle shell of the disk cartridge shown in FIG. 2; 
     FIG. 9 is a perspective view showing the state in which the opening portion is closed by closing the pair of shutter members attached to the middle shell of the disk cartridge shown in FIG. 2; 
     FIG. 10 is a perspective view showing the state in which a shutter mechanism is disposed on the lower shell of the disk cartridge shown in FIG.  2  and in which the opening portion is opened by opening the pair of shutter members; 
     FIG. 11 is a perspective view showing the state in which the shutter mechanism is disposed on the lower shell of the disk cartridge shown in FIG.  2  and in which the opening portion is closed by closing the pair of shutter members; 
     FIG. 12 is a plan view showing the disk cartridge shown in FIG. 11 from the upper direction; 
     FIG. 13 is a longitudinal cross-sectional view of the central portion of the disk cartridge shown in FIG.  2  and to which reference will be made in explaining a relationship between the disk cartridge and a turntable; 
     FIG. 14 is an explanatory diagram showing a relationship between operations of a cartridge housing and a shutter mechanism of the disk cartridge shown in FIG.  2  and illustrates the state in which the opening portion is completely closed by closing the pair of shutter members; 
     FIG. 15 is a plan view showing the outward appearance state of FIG. 14 from the lower shell side; 
     FIG. 16 is an explanatory diagram showing the state in which the middle shell is rotated a little from the state shown in FIG. 14 so that the pair of shutter members are opened a little; 
     FIG. 17 is an explanatory diagram showing the state in which the middle shell is further rotated a little from the state shown in FIG. 16 so that the pair of shutter members are further opened a little; 
     FIG. 18 is an explanatory diagram showing the state in which the middle shell is still further rotated from the state shown in FIG. 17 so that the pair of shutter members are still further opened a little; 
     FIG. 19 is an explanatory diagram showing the state in which the middle shell is still further rotated from the state shown in FIG.  18  so that the pair of shutter members are still further opened a little; 
     FIG. 20 is an explanatory diagram showing the state in which the middle shell is yet further rotated so that the pair of shutter members are opened completely; 
     FIGS. 21A to  21 C are diagrams to which reference will be made in explaining the manner in which the middle shell ascends and descends relative to the upper shell when the disk cartridge shown in FIG. 2 is opened and closed, wherein FIG. 21A is an exploded perspective view, FIG. 21B is a perspective view showing the state in which the middle shell descends, and FIG. 21C is a perspective view showing the state in which the middle shell ascends; 
     FIGS. 22A and 22B are diagrams to which reference will be made in explaining the manner in which the middle shell ascends and descends when the disk cartridge shown in FIGS. 21A to  21 C is opened and closed, wherein FIG. 22A is a cross-sectional view showing the state in which a cam portion of the middle shell is pressed against a lift up portion of the upper shell, and FIG. 22B is a cross-sectional view showing the state in which a cam portion of the middle shell is inserted into a cam groove of the upper shell; 
     FIG. 23 is an exploded perspective view showing a disk cartridge according to a second embodiment of the present invention and illustrating the disk cartridge from the lower surface side; 
     FIG. 24 is a perspective view showing the disk cartridge shown in FIG. 23 from the lower surface side and illustrates the state in which the opening portion is closed by closing the shutter mechanism; 
     FIG. 25 is a block diagram to which reference will be made in explaining a circuit arrangement of a disk recording and reproducing apparatus which uses the disk cartridge according to the present invention; 
     FIG. 26 is a perspective view showing the state presented before the disk cartridge shown in FIG. 2 is inserted into the disk recording and reproducing apparatus which uses the disk cartridge according to the present invention; and 
     FIG. 27 is a perspective view showing the state in which the disk cartridge shown in FIG. 2 is being inserted into the disk recording and reproducing apparatus which uses the disk cartridge according to the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A disk cartridge according to embodiments of the present invention will now be described below with reference to the accompanying drawings. 
     FIGS. 2 to  27  are diagrams showing disk cartridges according to the embodiments of the present invention. Specifically, FIG. 2 is an exploded perspective view showing a disk cartridge according to a first embodiment of the present invention from a lower shell side. FIG. 3 is a perspective view showing the same disk cartridge with its shutter being closed from the lower shell side. FIG. 4 is a perspective view showing the same disk cartridge with its shutter being opened. FIG. 5 is a bottom view of an upper shell. FIG. 6 is bottom view of a middle shell. FIG. 7 is a plan view of the lower shell. FIG. 8 is a perspective view showing the middle shell and a pair of shutter members being opened. FIG. 9 is a perspective view showing the state in which the pair of shutter members shown in FIG. 8 are being opened. FIG. 10 is a perspective view showing the lower shell and the pair of shutter members being opened. FIG. 11 is a perspective view showing the state in which the pair of shutter members shown in FIG. 10 are being closed. FIG. 12 is a plan view of FIG.  11 . FIG. 13 is an explanatory diagram showing the central portion in cross-section. 
     FIGS. 14 to  20  are explanatory diagrams showing a relationship between the rotated amount of the middle shell and the opened and closed states of the pair of shutter members, respectively. FIGS. 21A to  21 C are explanatory diagrams showing a relationship between the rotation of the middle shell and the ascending and descending operations, respectively. FIGS. 22A and 22B are respectively explanatory diagrams showing the state in which the shutter members are closely contacted/released based on the ascending and descending operations executed when the middle shell is rotated. FIG. 23 is an exploded perspective view showing a disk cartridge according to a second embodiment of the present invention from a lower shell side. FIG. 24 is a perspective view showing the same disk cartridge from the lower shell side. FIG. 25 is a block diagram to which reference will be made in explaining a circuit arrangement of an information disk recording and reproducing apparatus which can use the inventive disk cartridge according to an embodiment of the present invention. FIG. 26 is a perspective view showing the state presented before the disk cartridge according to the present invention is inserted into the disk recording and reproducing apparatus. FIG. 27 is a perspective view showing the state in which the disk cartridge is being inserted into the same disk recording and reproducing apparatus. 
     A disk cartridge  10  according to the first embodiment of the present invention stores therein a preformatted optical disk on which various information signals such as a music signal serving as audio information and a video signal and a music signal, both serving as video information, are recorded in advance or an optical disk (write once optical disk) on which an information signal such as audio information and video information can be recorded once or an optical disk (rewritable optical disk)  11  on which an information signal can be recorded many times repeatedly as a disk-like recording medium. However, the disk-like recording medium is not limited to the above optical disks, and a magnetic disk in which information can be memorized by magnetic status of a specific position on a magnetic thin layer formed on the surface of a thin disk, a magneto-optical disk in which information can be written in and read out from the magnetic thin layer formed similarly by using an optical head and a magnetic head and other storage mediums can be applied to the above disk-like recording medium. 
     As shown in FIGS. 2 to  4 , this disk cartridge  10  comprises a cartridge housing  12  formed by overlapping a pair of an upper shell  13 , a middle shell  14  and a lower shell  15 , the optical disk  11  stored within a disk compartment  16  formed within this cartridge housing  12  so as to become freely rotatable, a pair of shutter members  18   a ,  18   b  for opening and closing opening portions  25 ,  42  provided through the middle shell  14  and the lower shell  15 , and the like. In this disk cartridge  10 , when the optical disk  11  is in use in the horizontal direction, the upper shell  13  is normally disposed on the upper side. However, for convenience sake of explanation, the upper side and the lower side are reversed in FIG. 2, and hence the upper shell  13  is placed in the lower side. 
     The upper shell  13  is a thin disk-like member of substantially a square-shape, the front side of which is shaped as a circular arc as shown in FIGS. 2 and 5. At the central portion of the lower surface (i.e., shown as the surface in FIG. 2) of this upper shell  13 , there is provided a circular upper concave portion  19   a  into which substantially an upper half of the middle shell  14  in the thickness direction can be fitted so as to become rotatable. When the upper concave portion  19   a  is provided on the central portion of the lower surface, a front edge portion  13   a , left and right side edge portions  13   b ,  13   c  and a rear edge portion  13   d  are formed on the outer peripheral edge of the upper shell  13 . At substantially the central portions of the front edge portion  13   a  and the rear edge portion  13   d of the upper shell  13 , there are provided positioning concave portions  21   a  and  21   b  which are used to properly position the upper shell relative to the lower shell  15 . 
     23 On the outer peripheral edge of the upper concave portion  19   a  of the upper shell  13 , there are provided three circular-arc-like cam grooves  22 , each having a predetermined length, which are continued along the circumferential direction. Then, there are provided the three cam grooves  22 , whereby three lift up portions  23  (shown hatched as cross stripes in FIG.  6 ), each having a predetermined length, between the adjacent cam grooves  22  and  22 . A cam portion  46  provided in the middle shell  14  is slidably fitted into the three cam grooves  22 . 
     The lower shell  15  is overlaped to the lower surface of the upper shell  13 . As shown in FIGS. 2 and 7, the lower shell  15  has an outward appearance shape which is substantially similar to that of the upper shell  13  and is formed of a substantially square thin disk-like member whose front side is shaped as a circular arc. On the central portion of the flat surface (surface hidden as the back in FIG. 2) of this lower shell  15 , there is provided a circular lower concave portion  19   b  into which a substantially lower half portion in the thickness direction of the lower shell  15  is fitted rotatably. When this lower concave portion  19   b  is provided on the central portion of the flat surface of the lower shell, a front edge portion  15   a , left and right side edge portions  15   b ,  15   c  and a rear edge portion  15   d  are provided on the outer peripheral edge of the lower shell  15 . On substantially a central portion between the front edge portion  15   a  and the rear edge portion  15   d  of the lower shell  15 , there are provided positioning convex portions  24   a  and  24   b  which are used to properly position the lower shell relative to the upper shell  13 . 
     The convex portions  24   a ,  24   b  of the lower shell  15  are respectively engaged with the concave portions  21   a ,  21   b  of the upper shell  13 , whereby the upper and lower shells  13 ,  15  are properly positioned and overlaped together in a predetermined state. The middle shell  14  is stored within a circular space formed of the upper and lower concave portions  19   a ,  19   b  formed within the upper and lower shells  13 ,  15  in such a manner that it may become freely rotatable. An opening portion  25  extended in the front and back direction is provided at the central portion of the lower shell  15 . This opening portion  25  is used to allow a turntable of a disk rotary drive mechanism and an optical head of an optical pickup device to face the optical disk  11  and is set to be wide enough so that they can be inserted into and ejected from the opening portion. 
     Specifically, the opening portion  25  of the lower shell  15  includes a table opening portion  25   a  set at the central portion into and from which the turntable is inserted and ejected and a head opening portion  25   b  extended in the front and back direction of the table opening portion  25   a . The shape of this opening portion  25  is devised such that two optical heads can be simultaneously inserted into and ejected from both sides of the table opening portion  25   a  in the head opening portion  25   b . On the inner surfaces of both ends of the front and back direction of the head opening portion  25   b , there are respectively provided cover portions  26   a ,  26   b  which are used to close the spacing between them and shutter members  18   a ,  18   b  which will be described later on. Further, within the lower concave portion  19   b , there are provided a pair of operational convexs  27   a ,  27   b  which become symmetrical with respect to the table opening portion  25   a.    
     The middle shell  14  is stored within the upper and lower concave portions  19   a ,  19   b  formed by the upper shell  13  and the lower shell  15  having the above arrangements, and the middle shell  14  and the upper shell  13  constitute a disk compartment  16  formed of a circular spacing portion. Within this disk compartment  16 , there is stored the optical disk  11  with a predetermined spacing in the outside of the radius direction and in the thickness direction. This optical disk  11  is formed of a thin disk-like recording member having a center hole  11   a  bored at the central portion. 
     To one surface of the central portion of this optical disk  11 , there is fixed a center hub  28  so as to cover the center hole  11   a  by a fixing means such as an adhesive agent. This center hub  28  is formed of a magnetic material such as an iron plate attracted by magnetic force from a magnet (not shown). A turntable of a disk rotary drive apparatus incorporated within a disk recording and reproducing apparatus is fitted through the center hole  11   a  to the center hub  28 . The center hub  28  is attracted by magnet force from the magnet provided on the turntable. Consequently, the optical disk  11  is chucked to the turntable and integrated in the rotation direction, whereby the optical disk  11  is rotated at a predetermined velocity (e.g., at a constant linear velocity) by driving a spindle motor (not shown) to which the turntable is attached. 
     On one surface of this optical disk  11 , there are provided an information recording area  29   a  on which information can be recorded and which is opposed by an optical head of an optical pickup device incorporated within the disk recording and reproducing apparatus and a non-recording area  29   b  on which information cannot be recorded. The non-recording area  29   b  is comprised of an inner non-recording area  29   b  having a predetermined width provided on the outside of the center hub  23  which is in the inside of the optical disk  11  and an outer non-recording area  29   b  having a predetermined width provided on the outer peripheral edge of the optical disk  11 . An information recording area  29   a  is provided between the inner and outer non-recording areas  29   b  and  29   b . As a material of a substrate of this optical disk  11 , synthetic resins such as polycarbonate (PC) and polyolefin should be suitably used. It is needless to say that other synthetic resins can be used and that the above materials are not limited to the synthetic resins and that various materials such as a glass material and an aluminum alloy which can be used as an information recording medium may be used. 
     An opening window  30  is bored at substantially central portions of one side edge portions  13   b ,  15   b  of the upper and lower shells  13 ,  15  so as to expose part of the outer peripheral surface of the middle shell  14 . This opening window  30  is comprised of an upper recess portion  30   a  provided on the joint portion of the upper shell  13  and a lower recess portion  30   b  provided on the joint portion of the lower shell  15 . Further, on one side edge portions  13   b ,  15   b  of the upper and lower shells  13 ,  15 , there is provided a guide groove  31  which is extended along the joint surface in the front and back direction. This guide groove  31  is provided in order to prevent a mis-insertion from being carried out when the disk cartridge  10  is loaded onto the disk recording and reproducing apparatus. The opening window  30  is communicated with the guide groove  31 .  31  On the front corner portions of one side edge portions  13   b ,  15   b of the upper and lower shells  13 ,  15 , there is provided a lock storing portion  33  in which a lock member  32  can be stored rotatably. This lock storing portion  33  is communicated with the concave portions  19   a ,  19   b  and is communicated with the guide groove  31  through an opening hole  34  bored through one side edge portions  13   b ,  15   b . Further, on the lock storing portion  33  of the upper shell  13 , there is provided a support shaft  35  which rotatably supports the lock member  32  in such a manner that it may be protruded from the lower shell  15  side. 
     This lock member  32  is formed of a lever-like member which is rotatably fitted into a supporting shaft  35  in such a manner that it can be swung in the plane direction as shown in FIGS. 2,  14  and the like. A stopper portion  36  having a plurality of teeth is provided at the lock member  32  at its one end in the longitudinal direction. Further, the lock member  32  has a spring piece  38  integrally formed therewith and which is protruded in the same direction so as to oppose to the stopper portion  36 . 
     The lock member  32  having the above arrangement is fitted into the supporting shaft  35  when the stopper portion  36  is faced toward the concave portions  19   a ,  19   b  side. The spring piece  38  is brought in contact with the inner surfaces of the front edge portions  13   a ,  15   a . Under spring force of this spring piece  38 , the stopper portion  36  is spring-biased toward the concave portions  19   a ,  19   b  side. Concurrently therewith, the input portion  37  is extended through the opening hole  34  from the inside and is protruded into the guide groove  31 . 
     The middle shell  14  which is rotatably fitted into the upper and lower concave portions  19   a ,  19   b  of the upper shell  13  and the lower shell  15  has the arrangement shown in FIGS. 2,  6  and the like. This middle shell  14  includes a flat surface portion  40  formed of a disk-like thin plate material and a ring portion  41  continuously formed at the outer peripheral edge of the flat surface portion  40 . The flat surface portion  40  of the middle shell  14  is comprised of a pair of semicircle pieces  40   a ,  40   a  formed as substantially semicircular shapes. An opening portion  42  is formed by forming a spacing between chord sides of the two semicircle pieces  40   a ,  40   a . This opening portion  42  has a shape and a size equal to those of the opening portion  25  of the lower shell  15 . 
     Specifically, similarly to the opening portion  25 , the opening portion  42  includes a table opening portion  42   a  set at the central portion of the flat surface portion  40  and into and from which the turntable is inserted and ejected and head opening portions  42   b ,  42   b set on both sides in the diametrical direction of the table opening portion  42   a  and to which optical heads are opposed. On the inner peripheral edge of the flat surface portion  40   a  which surrounds the table opening portion  42   a , there is provided a supporting edge portion  40   b  which supports from the lower direction the inner non-recording area  29   b  of the optical disk  11  when it is in a state of being free. 
     On the outer peripheral surface of the ring portion  41  of the middle shell  14 , there is provided a gear portion  43  having a large number of gear teeth over a predetermined range of the peripheral direction. As shown in FIG. 14, in assembling the cartridge housing, when the opening portion  42  of the middle shell  14  is made coincident with the opening portion  25  of the lower shell  15 , this gear portion  43  is set at the position in which it is opposed to the opening window  30 . The range of this gear portion  43  is set to be an angular range slightly larger than the rotation angle of the middle shell  14 . Specifically, as shown in FIGS. 16 to  20 , when the pair of shutter members  18   a ,  18   b  are completely closed such that the opening portion  42  is inclined by a maximum inclination angle relative to the opening portion  25 , one end of the circumferential direction of the gear portion  43  is exposed to the opening window  30 . Then, when the pair of shutter members  18   a ,  18   b  are completely opened such that the middle shell  14  is rotated by a predetermined angle to cause the opening portion  42  to overlap the opening portion  25 , the other end of the circumferential direction of the gear portion  43  is exposed to the opening window  30 . 
     On the outside of one end of the gear portion  43  of this middle shell  14 , there is provided a stopper portion  44  for limiting the rotation movement amount of the middle shell  14  in such a manner that it may be protruded toward the outside of the radius direction. Since the gear portion  43  and the stopper portion  44  are projected from the outer peripheral surface of the ring portion  41  to the outside as described above, relief grooves  45   a ,  45   b  are respectively provided on the upper shell  13  and the lower shell  15  at their corresponding portions to avoid contacting with the stopper portion  44  and the like such that they can be allowed to pass. Further, on one end face of the ring portion  41 , there are provided three circular-arc-like cam portions  46  (portions shown by a large number of dots in FIG.  6 ), each having a predetermined length, continued in the circumferential direction. These cam portions  46  are respectively engaged with the three cam grooves of the upper shell  13  while the middle shell  14  is assembled on the upper shell  13 . When this middle shell  14  is rotated at a predetermined angle, the respective cam portions  46  are overlaid on the three lift up portions  23 , respectively. 
     On the other end face of the ring portion  41  of the middle shell  14 , there are provided two circular-arc-like protruded rail portions  47  which are protruded on the opposite side of the cam portions  46 . These protruded rail portions  47 ,  47  are used to restrict the pair of shutter members  18   a ,  18   b  not to move to the outside of the radius direction. To avoid the contact with these protruded rail portions  47 ,  47 , one cover portions  26   a ,  26   a  of the upper shell  13  have relief grooves  48  extended in the circumferential direction, respectively as shown in FIG.  7 . Further, on the flat surface portion  40  of the middle shell  14 , there are respectively provided a pair of supporting apertures  49 ,  49  which are used to support the pair of shutter members  18   a ,  18   b  such that the pair of shutter members become freely rotatable in the plane direction. The pair of supporting apertures  49 ,  49  are each set in one edge portions of the head opening portions  42   b ,  42   b  with the table opening portion  42   a  as the center in a point-symmetrical fashion. 
     A shutter mechanism  18  that is attached to the middle shell  14  through the pair of supporting apertures  49 ,  49  is comprised of the pair of shutter members  18   a ,  18   b  which are the same in shape and size. The pair of shutter members  18   a ,  18   b  have the shape and structure shown in FIGS. 8 and 9. Specifically, the pair of shutter members  18   a ,  18   b  are each formed of substantially a semicircular thin plate material. At substantially a central portion of a chord side of each of the shutter members  18   a ,  18   b , there are provided stepped portions  50  having a predetermined length in the direction perpendicular to the normal. This stepped portion  50  forms at its both sides a convex-side joint portion  50   a  and a concave-side joint portion  50   b.    
     On the respective joint portions  50   a ,  50   b  of the shutter members  18   a ,  18   b , there are respectively provided overhang portions  51 ,  51  which are overhung in the direction perpendicular to the direction in which the chords are extended. Thus, in the pair of shutter members  18   a ,  18   b , the end face of the convex-side joint portion  50   a and the end face of the concave-side joint portion  50   b  are respectively opposed to each other, whereby the overhang portion  51   a of the convex-side joint portion  50   a  are overlaped to the overhang portion  51   b  of the concave-side joint portion  50   b . Shaft portions  52  which are projected toward the overhang portions  51  side are respectively attached to shaft attachment portions  50   c  which are end portions of the convex-side joint portion  50   a  side of the chord sides of the shutter members  18   a ,  18   b.    
     The shaft portions  52  are fitted into the supporting apertures  49  of the flat surface portion  40 , whereby the respective shutter members  18   a ,  18   b  are rotatably attached to the middle shell  14 . In this case, the pair of shutter members  18   a ,  18   b  are attached to the middle shell in such a manner that their chord sides are opposed to each other. As a result, as shown in FIG. 8, when the pair of shutter members  18   a ,  18   b  are rotated so as to become distant from each other on the outside, respectively, the respective shutter members  18   a ,  18   b  are mounted on the semicircular pieces  40   a  such that they are hidden by the semicircular pieces. On the other hand, when the pair of shutter members  18   a ,  18   b  are inwardly rotated such that their joint portions  50   a ,  50   b  are brought in contact with each other, a substantially circular cover portion is formed at the central portion of the opening portion  42  by the pair of shutter members  18   a ,  18   b  as shown in FIG.  9 . 
     Further, on the concave-side joint portions  50   b  of the shutter members  18   a ,  18   b , there are respectively provided open and close grooves  53  which are used to open and close the shutter members  18   a ,  18   b . The open and close grooves  53  are extended in the radial direction from the shaft portions  52  of the shutter members  18   a ,  18   b . The operational convexs  27   a ,  27   b  of the lower shell  15  are slidably engaged with the open and close grooves  53 . Further, at the end portions on the outsides of the respective open and close grooves  53 , there are provided resilient pieces  54  formed by cutting and concave portions  53   a  which are used to relieve the operational convexs  27   a ,  27   b . On the circular-arc-like sides of the shutter members  18   a ,  18   b , there are provided close pieces  55  which are used to close openings left after the shaft portions  52  had been moved while the shutter is being closed. 
     The upper shell  13 , the middle shell  14 , the lower shell  15  and the shutter members  18   a ,  18   b  having the above arrangements may be made of synthetic resins such as ABS resin (acrylonitrile-butadiene-styrene resin) and HIPS (high impact polystyrene). However, it is needless to say that they may be made of other synthetic resins and metal materials such as an aluminum alloy and a stainless steel. 
     The above middle shell  14 , the operational convexs  27   a ,  27   b , a rack rod  70  and the like constitute a shutter opening and closing mechanism. Then, the supporting apertures and the shaft portions  52  constitute a rotary coupling portion, and the operational convexs  27   a ,  27   b  and the open and close grooves  53  constitute a movement coupling portion. Although not shown, the cartridge housing  12 , for example, of the disk cartridge  10  has at its rear corner portion a write-protect mechanism for preventing information recorded on a recording medium such as an optical disk from being erased by mistake. Such write-protect mechanism can be comprised of an operation window opened on the back of the cartridge housing  12 , a write-protect piece attached so as to open and close this operation window and the like. 
     The disk cartridge  10  having the above arrangement can be assembled with ease as follows, for example. As shown in FIG. 2, an assembly work of this disk cartridge  10  is executed in the state in which the upper shell  13  is disposed in the lower direction. First, the optical disk  11  is installed within the upper-side concave portion  19   a  of the upper shell  13 . At that very moment, the optical disk  11  is inserted into the upper-side concave portion  19   a  while its surface with the center hub  28  is being faced to the lower direction. Next, the opening side of the middle shell  14  is fitted into the upper-side concave portion  19   a  to cover the optical disk  11 , whereby the optical disk  11  is rotatably stored within the disk compartment  16  comprised of the middle shell  14  and the upper shell  13 . At that very moment, the direction in which the opening portion  42  of the middle shell  14  is extended is made coincident with the front and back direction of the upper shell  13  and one end of the gear portion  43  is faced to the opening window  30 . 
     Next, the shutter mechanism  18  is assembled on the middle shell  14 . In this assembly work, while the chord sides of the pair of shutter members  18   a ,  18   b  are being opposed to each other, the shaft portions  52  are respectively fitted into the supporting apertures  49  of the flat surface portion  30  in this state. Then, the respective shutter members  18   a ,  18   b  are separated from each other and are thereby overlapped on the semicircular pieces  40   a  of the middle shell  14 . Concurrently therewith or before or after the above assembly work, the lock member  32  is attached to the supporting shaft  35  of the lock storage portion  33 . In that case, the spring piece  38  of the lock member  32  is brought in contact with the inner surface of the front edge portion  13   a  of the upper shell  13 , and the input portion  37  is projected from the opening hole  34  into the guide groove  31  under spring force of this spring piece  38 . 
     Next, the lower shell  15  is laid over the middle shell  14  including the shutter mechanism  18 , and the lower shell  15  is overlaped to the upper shell  13 . At that time, the positioning convex portions  24   a ,  24   b  of the lower shell  15  are fitted into the positioning concave portions  21   a ,  21   b  of the upper shell  13 , whereby the lower shell  15  is automatically and properly positioned relative to the upper shell  13 . In that case, when the pair of shutter members  18   a ,  18   b  are disposed at the above positions, the pair of operational convexs  27   a ,  27   b  provided within the lower-side concave portion  19   b  of the lower shell  15  can be respectively opposed to the pair of open and close grooves  53 . Therefore, only by overlapping the lower shell  15  on the upper shell  13  without considering the positions of the pair of open and close grooves  53 , the pair of operational convexs  27   a ,  27   b  can be engaged with the pair of open and close grooves  53 . 
     Thereafter, the lower shell  15  is overlaped to and fixed to the upper shell  13  by using a plurality of fixing screws. Thus, the assembly work of the disk cartridge  10  is completed. In this case, without using a fixing means comprised of independent members such as fixing screws, the joint surfaces of the upper shell  13  and the lower shell  15  can directly be joined by using a suitable means such as an adhesive agent. As described above, according to the disk cartridge of this embodiment, the number of assemblies can be decreased and the assembly work can be carried out extremely easily. 
     In this case, the joint portions  50   a ,  50   b  of the pair of shutter members  18   a ,  18   b  have the overhang portions  51   a ,  51   b , each having about half thickness and these overhang portions  51   a ,  51   b  are inserted beneath the other joint portions  50   a ,  50   b , respectively. Therefore, since the overall arrangements of the joint portions  50   a ,  50   b  are properly overlapped with each other, a labyrinth effect can be achieved so that the disk cartridge can be prevented from being smudged by dusts and the like over the whole length of the joint portions  50   a ,  50   b . In addition, when the joint portions  50   a ,  50   b  are pressed inwardly, external force applied to the joint portions  50   a ,  50   b  is received by the overhang portions  51   a ,  51   b  located at the inside. Consequently, the overhang portions  51   a ,  51   b  the engagement state between the joint portions  50   a  and  50   b  can be strengthened more by the overhang portions  51   a ,  51   b , whereby the shutter members can be prevented from being deformed and the disk cartridge can be prevented more reliably from being smudged by dusts and the like. 
     FIG. 3 shows the state presented before the disk cartridge  10  having the above arrangement is loaded on the disk recording and reproducing apparatus. Specifically, the pair of shutter members  18   a ,  18   b  of the shutter mechanism  18  completely close the opening portion  25  of the lower shell  15  and the opening portion  42  of the middle shell  14  due to its own tare and frictional force generated between the lower shell  15  and the middle shell  14 . Further, the three cam portions  46  of the middle shell  14  are projected from the three cam grooves  22  of the upper shell  13  and are thereby laid over the lift up portions  23 . As a consequence, the pair of shutter members  18   a ,  18   b  are sandwiched from both sides by the upper shell  13  and the middle shell  14 . 
     Further, as shown in FIG.  14  and the like, the stopper portion  36  is engaged with the gear portion  43  of the middle shell  14  under spring force of the spring piece  38  of the lock member  32 . Consequently, the pair of shutter members  18   a ,  18   b  are doubly locked by a holding mechanism based on cam elevation operations and a lock mechanism based on the lock member  32 . Accordingly, the state in which the opening portions  25 ,  42  are closed by the shutter mechanism  18  can be held with high reliability and the shutter mechanism  18  can reliably be prevented from being opened by mistake due to some cause such as vibrations. 
     When the disk cartridge  10  in this state is inserted into a disk recording and reproducing apparatus  60  as shown in FIGS. 26 and 27, the lock member  32  is released from the locked state by a shutter opening and closing mechanism provided in this disk recording and reproducing apparatus  60 . Thereafter, this shutter opening and closing mechanism opens the shutter mechanism  18  to open the opening portions  25 ,  42 . The manner in which this shutter mechanism  18  is opened and closed will be described in detail later on. 
     Next, the disk recording and reproducing apparatus  60  will be described. As shown in FIGS. 26 and 27, this disk recording and reproducing apparatus  60  comprises an outer case  61  formed of a hollow housing, a recording and reproducing apparatus body  62  stored within this outer case  61  and the like. The outer case  61  includes a case body  63  which is opened in the upper surface and the front, a case lid  64  detachably attached to the upper portion so as to close the upper surface of the case body  63 , a front panel  65  detachably attached to the front portion so as to close the fronts of the case body  63  and the case lid  64  and the like. 
     The case body  63  is provided with four leg assemblies  63   a  which are projected in the lower direction. The disk recording and reproducing apparatus  60  is supported by these leg assemblies  63   a and thereby attached to the proper position. The front panel  65  is formed of an oblong plate-like member, and an oblong cartridge slot  66  is formed on the upper portion of the front panel. The cartridge slot  66  has approximately the same dimension as that of the front side of the disk cartridge  10 . This cartridge slot  66  is constantly closed by an open and close lid  67  disposed on the inside of the cartridge slot. 
     This open and close lid  67  is pressed by the front portion of the disk cartridge  10  and the disk cartridge is inserted into the cartridge slot at its predetermined position as shown in FIG.  27 . As a result, the disk cartridge  10  is automatically loaded into the disk recording and reproducing apparatus by a loading mechanism not shown. Then, the disk cartridge  10  transported by the loading mechanism is properly located at a predetermined position and fixed thereto within the disk recording and reproducing apparatus  60 . Concurrently therewith or before and after the above operation, the shutter opening and closing mechanism provided in the disk recording and reproducing apparatus  60  operates the shutter mechanism  18  to open the opening portions  25 ,  42 . 
     Thereafter, a turntable  68   a  of a disk rotary drive apparatus  68  and an optical head  69   a  of an optical pickup device  69  are entered into the opening portions  25 ,  42  thus opened. Then, the turntable  68   a  is engaged with the center aperture  11   a  of the optical disk  11  and the center hub  28  is attracted and held by a magnet incorporated within the turntable  68   a . Concurrently therewith or before and after the above operation, the optical head  69   a  is approached to a predetermined position of the information recording area  29   a  of the optical disk  11 , whereby the disk recording and reproducing apparatus  60  becomes able to record an information signal on the optical disk  11  and to reproduce a recorded information signal from the optical disk. 
     The recording and reproducing apparatus body  62  of the disk recording and reproducing apparatus  60  comprises constituents shown in FIG. 25, for example. Specifically, the recording and reproducing apparatus body  62  includes three control apparatus of a system controller S 1 , a memory controller S 5  and a drive controller D 1 . The system controller S 1  and the memory controller S 5  are directly connected, and a read-only memory apparatus (ROM (read-only memory)) S 2  and a read and write memory apparatus (RAM (random-access memory)) S 3  are connected to a connection line between the system controller and the memory controller. Further, a memory S 4  and MPEG  2  (moving picture compression system applied to existing television broadcasting capable of 3 M to 40 Mbps, HDTV (high definition television) and broadband ISDN (integrated services digital network)) encoder S 6  and decoder S 7  are connected to the memory controller S 5 . Further, a control panel S 8  and a remote control receiving section S 9  are connected to the system controller S 1 . 
     An ECC D 4  is connected to the drive controller D 1 , and a memory apparatus (ROM) D 2 , a memory apparatus (RAM) D 3 , a servo circuit D 6  and an address decoder D 7  are connected to a connection line between the drive controller D 1  and the ECC D 4 . Further, the drive controller D 1  is connected with a recording error judgment circuit D 8  for judging whether or not an error occurred upon recording. This drive controller D 1  is connected through a command interface to the system controller S 1 . The drive controller is connected through the ECC D 4  and a data interface to the memory controller S 5  and a modem (modulator and demodulator) circuit D 5 . Then, the modem circuit D 5  is also connected to the optical head  69   a  of the optical pickup device  69 . 
     The servo circuit D 6  is connected to a spindle motor  68  which is serving as a disk rotary drive apparatus, the optical pickup device  69  and the recording error judgment circuit D 8 . Further, the optical pickup device  69  is connected to an address decoder D 7 , and this address decoder D 7  is also connected to the recording error judgment circuit D 8 . 
     When the disk cartridge  10  is inserted into the disk recording and reproducing apparatus  60  having the above arrangement from the cartridge slot  66 , as shown in FIG. 14, a rack rod  70  which shows a practical example of the shutter opening and closing mechanism incorporated within the disk recording and reproducing apparatus  60  is entered into the guide groove  31  formed on one side portion of the disk cartridge  10 , whereby the input portion  37  of the lock member  32  projected into the guide groove  31  is pushed into the lock storage portion  33  by pressing force of the rack rod  70  against spring force of the spring piece  38 . Further, when the rack rod  70  is advanced relatively, the tip end of the gear portion  70   a  is meshed with the gear portion  43  of the middle shell  14 . 
     This gear portion  43  is meshed with the gear portion  70   a  of the rack rod  70 , whereby the middle shell  14  is rotated in response to the moved amount of the rack rod  70  as shown in FIGS. 14 to  20 . FIGS. 14 and 15 show the state presented before the gear portion  70   a of the rack rod  70  is meshed with the gear portion  43  of the middle shell  14 . In this state, the convex-side joint portion  50   a  and the concave-side joint portion  50   b  of the pair of shutter members  18   a ,  18   b  are pressedagainst with each other and are thereby closely contacted with each other. At that very time, since the opening portion  42  of the middle shell  14  is located at the inclined position relative to the opening portion  25  of the lower shell  15 , although only the central portion in which the two opening portions  25 ,  42  are overlapped with each other is opened, that opening portion is closed by the pair of shutter members  18   a ,  18   b . Accordingly, the disk compartment  16  can be prevented from being smudged by dusts and the like from the overlapping portion of the opening portions  25 ,  42 . 
     The shaft attachment portions  50   c  of the pair of shutter members  18   a ,  18   b  are interposed at the portions in which the opening portions  25  and  42  are close to each other with an acute angle, and the shaft attachment portion  50   c  is located at the inside of one cover portion  26   b , thereby removing the space. The closing piece  55  is interposed at the inside of the other cover member  26   b , and this closing piece  55  is located at the inside of the other cover member  26   a , thereby removing the space. Further, the open and close grooves  53  provided on the pair of shutter members  18   a ,  18   b  are respectively closed by the lower shell  15 . Therefore, since all spaces from the opening portion  25  of the lower shell  15  to the opening portion  42  of the middle shell  14  are completely closed by the pair of shutter members  18   a ,  18   b , the disk compartment  16  can be prevented from being smudged by dusts and the like through the opening portions  25 ,  42 . 
     In this state, when the rack rod  70  is being entered into the guide groove  31  as the disk cartridge  10  is being inserted into the disk recording and reproducing apparatus and the gear portion  70  and the gear portion  43  of the middle shell  14  commence to be meshed with each other, the middle shell  14  is rotated in the counter-clockwise direction in FIG.  14 . In this case, in the initial state in which the middle shell  14  commences to rotate, as shown in FIGS. 21C and 22A, the cam portion  26  is laid over the lift up portion  23  and sandwiches the pair of shutter members  18   a ,  18   b  between it and the upper shell  13  so that relatively large force is required to rotate the middle shell  14 . When the middle shell  14  is rotated against large frictional force generated from the cam portion  46  so that the cam portion  46  passes the lift up portion  23  and enters the cam groove  22  as shown in FIGS. 21B and 22B, frictional force of the cam portion  46  is lost. Consequently, the middle shell  14  will be able to rotate extremely lightly and smoothly. 
     At that time, while the respective shaft portions  52  of the pair of shutter members  18   a ,  18   b  are similarly rotated as the middle shell  14  is rotated, the operational convexs  27   a ,  27   b  of the lower shell  15  are slidably engaged with the open and close grooves  54  provided on the other end side. The pair of operational convexs  27   a ,  27   b  are provided on the lower shell  15  and the operational convexs  27   a ,  27   b  are never moved. On the other hand, the open and close grooves  53  are extended in the direction S connecting the shaft portion  52  and the operational convex  27   b  (or  27   a ) in such a manner that they can be relatively moved within the operational convexs  27   a ,  27   b  as the shutter members  18   a ,  18   b  are rotated. 
     As a result, the operational convex  27   b  (or  27   a ) is moved within the open and close groove  53  toward the shaft portion  52  side in response to the rotation amounts of the shutter members  18   a ,  18   b . Thus, in each of the shutter members  18   a ,  18   b , the shaft portion  52  is moved toward the corresponding operational convex  27   b  ( 27   a ) side. Consequently, the respective shutter members  18   a ,  18   b  are moved on the corresponding semicircular pieces  40  from the state shown in FIG.  16  through the states shown in FIGS. 17,  18  and  19  as shown in FIG.  20 . Thus, the respective shutter members  18   a ,  18   b  are overlapped on the semicircular pieces  40   a  so that the upper and lower opening portions  25 ,  42  are opened completely as shown in FIG.  10  and the like. Therefore, a part of the optical disk  11  stored within the disk compartment  16  is exposed through the opening portions  25 ,  42 . 
     As a consequence, as shown in FIG. 13, the turntable  68   a  and the optical head  69   a  can be inserted into the opening portions  25 ,  42 . Accordingly, the turntable  68   a  or the like is moved toward the disk cartridge  10  side or the disk cartridge  10  is moved toward the turntable  68   a  side, whereby the turntable  68   a  is entered into the table opening portion  25   a  provided at the central portion and the optical head  69   a  is entered into the head opening portion  25   b provided at both sides. As a result, the optical disk  11  is loaded on and chucked to the turntable  68   a  and the optical head  69   a  is opposed to the information recording area  29   a  of the optical disk  11  with a predetermined space with the result that the disk recording and reproducing apparatus  60  becomes able to reproduce or record an information signal from or on the information recording area  29   a  of the optical disk  11 . 
     Accordingly, the recording and reproducing apparatus body  62  is operated, the optical disk  11  is rotated by driving the spindle motor  68  through the turntable  68   a , laser light is emitted from the optical head  69   a  by driving the optical pickup device  69  and the information signal can be read out from the information recording area  29   a  or a new information signal can be recorded on the information recording area  29   a . Thus, the recording and reproducing apparatus body  62  can reproduce or record the information signal. 
     After the information signal had been reproduced or recorded as described above, when a user operates a cartridge eject button (not shown) provided on the disk recording and reproducing apparatus  60 , for example, the disk cartridge  10  is ejected from the disk recording and reproducing apparatus  60  through the operation of the loading mechanism. Specifically, when the disk cartridge  10  is moved toward the cartridge slot  66  side by the eject operation of the loading mechanism, the rack rod  70  is retreated relatively. 
     When the rack rod  70  is retreated, the middle shell  14  and the pair of shutter members  18   a ,  18   b  are operated in the manner opposite to the above manner in which the optical disk is inserted into the recording and reproducing apparatus. Specifically, in the state in which the pair of shutter members  18   a ,  18   b  are completely opened as shown in FIG. 20, the middle shell  14  and the pair of shutter members  18   a ,  18   b  are rotated in the clockwise direction and the middle shell and the pair of shutter members are changed to the state shown in FIG.  16  through the states shown in FIGS. 19,  18  and  17 . In the states shown in FIGS. 20 to  16 , the cam groove  46  of the middle shell  14  exists within the cam groove  22  so that the middle shell  14  can be rotated by small force F. 
     Next, when the middle shell  14  is changed from the state shown in FIG. 16 to the state shown in FIG. 14, since the cam groove  23  is brought in contact with the lift up portion  23  and then laid over the lift up portion  23  so that the above frictional force is applied to the rotation of the middle shell  14 . Then, immediately before the closed state shown in FIG. 14, the operational convex  27   b  (or  27   a ) moves the open and close groove  53  to the outermost portion so that the open and close groove comes in contact with and presses the resilient piece  54 . As a result, spring force X is generated in the resilient piece  54  by pressing force of the operational convex  27   b (or  27   a ). Under spring force X of this resilient piece  54 , urging contact force Y which is acting on the other shutter member  18   a  (or  18   b ) is generated in the central stepped portion  50 . 
     Since this urging contact force Y is generated in both of the pair of shutter members  18   a ,  18   b , the airtightness of the pair of shutter members  18   a ,  18   b  can be improved more by their urging contact force Y. Accordingly, the disk compartment  16  can be more effectively prevented from being smudged by dusts and the like by improving the dustproof property at the joint portion of the pair of shutter members  18   a ,  18   b.    
     Thereafter, aft the gear portion  70   a  of the rack rod  70  had been disengaged from the gear portion  43 , the rack rod  70  is passed through the opening hole  34 , whereby the input portion  37  of the lock member  32  is projected from the opening hole  34  into the guide groove  31 . Concurrently therewith, the stopper portion  36  is moved to the inside and is thereby meshed with the gear portion  34 . As a result, the middle shell  14  is locked by the lock member  32  and its rotation is thereby stopped. Then, as shown in FIGS. 11 and 12, the whole of the opening portions  25 ,  42  is completely closed by the pair of shutter members  18   a ,  18   b.    
     FIGS. 23 and 24 show a disk cartridge according to a second embodiment of the present invention. A disk cartridge  80  shown in this embodiment is of one-head type disk cartridge for use with a disk recording and reproducing apparatus using one head. As will be clear from FIG. 23, this disk cartridge  80  differs from the disk cartridge  10  shown in FIG.  2  and the like only in a lower shell  85 . Therefore, elements and parts identical to those of FIG. 2 are marked with the identical reference numerals and need not be described. Only an opening portion  86  of the lower shell  85  having different arrangement will be described. 
     The opening portion  86  of the lower shell  85  has the same shape and arrangement as those of the opening portion  25  of the above lower shell  15  from which only one head opening  25   b  is removed. Other shapes and arrangements are the same. The one head disk cartridge  80  can easily be manufactured by using the one head lower shell  85  instead of the 2-head lower shell  15 , and hence the present invention can easily cope with a one-head type disk recording and reproducing apparatus and a two-head type disk recording and reproducing apparatus. In addition, according to this embodiment, the manufacturing and assembling lines can be made common and the present invention can properly cope with users&#39; needs. 
     While the present invention has been described so far, the present invention is not limited to the above embodiments. For example, while the optical disk has been used as the information recording medium as described above, the present invention is not limited thereto and can be applied to magnetic disks such as a magneto-optical disk and a floppy disk and other various kinds of disk-like recording mediums. Further, while the present invention has been applied to the disk recording and reproducing apparatus for both of recording and reproduction as the information recording and reproducing apparatus as described above, the present invention is not limited thereto and can, of course, be applied to a disk recording apparatus or a disk reproducing apparatus which is capable of either recording or reproducing a disk. 
     While the opening portions  42 ,  25  are provided on the middle shell  4  and the lower shell  15  of the cartridge housing  12  as described above, the present invention is not limited thereto and the opening portion may be provided on the upper shell, for example, so that the disk can be accessed simultaneously from upper and lower directions of the cartridge housing  12 . Further, the disk compartment may be comprised of the middle shell and the lower shell, the disk-like recording medium may be stored within the disk compartment and the opening portion may be provided on the upper shell. As described above, the present invention may be variously modified without departing from the gist thereof. 
     As described above, according to the disk cartridge of the present invention, in the disk cartridge including the cartridge housing having the disk compartment, the disk-like recording medium and the pair of shutter members, since the opening portion is extended in the diametrical direction of the disk-like recording medium through the central portion of the cartridge housing, two recording and/or reproducing head portions can simultaneously be inserted into the opening portion so that two recording and/or reproducing operations can be executed at the same time. Furthermore, the two recording and/or reproducing head portions can approach the innermost periphery of the disk-like recording medium and therefore, the disk cartridge can approach the innermost periphery of the disk-like recording medium with a high efficiency. 
     Accordingly, for example, while new information is being recorded on the disk-like recording medium, information that was recorded immediately after new information has been recorded can be reproduced for confirmation. Alternatively, one data can be divided and divided data can simultaneously be recorded on different areas of the disk-like recording medium, thereby making it possible to increase a data rate. Furthermore, the disk cartridge according to the present invention can be applied to a system apparatus in which two optical heads are disposed on the plane relative to the disk-like recording medium, and therefore there can be provided a two-head type disk cartridge which can meet with such requirements. 
     According to the disk cartridge of the present invention, since the cartridge housing includes the middle shell in addition to the upper shell and the lower shell, the middle shell is rotatably supported by the upper shell and the lower shell and the opening portion is bored through the middle shell and the upper shell or the lower shell, there can be formed the opening portion into and from which two recording and/or reproducing head portions can be inserted and ejected simultaneously. There can be provided the disk cartridge in which the opening portion can be opened and closed by the pair of shutter members. 
     According to the disk cartridge of the present invention, since the pair of shutter members are comprised of the combination of two substantially semicircular same plate materials, the pair of shutter members are symmetrically disposed across the opening portion, the guide grooves are provided on one side portions of the respective shutter members, the pivots are provided on the other side portions and the pivots are supported to the middle shell so as to become freely rotatable, the opening portion of the wide range can be opened and closed while the areas of the shutter members can be reduced. Therefore, an efficiency with which the opening portion is opened and closed can be increased, and the whole of the disk cartridge can be made compact and thin. 
     According to the disk cartridge of the present invention, since the pair of guide grooves are provided on one of the pair of shutter members and the cartridge housing and the pair of operational convexs are provided on the other of the pair of guide grooves and the guide grooves are moved along the operational convexs based on the rotation of the middle shell, the pair of shutter members can be moved with a high reliability so that the opening portion can be opened and closed smoothly and reliably. 
     According to the disk cartridge of the present invention, in the disk cartridge comprising the cartridge housing having the disk compartment, the disk-like recording medium and the pair of shutter members, since the disk cartridge includes the shutter opening and closing mechanism for moving the pair of shutter members based on the rotation of the middle shell so that the end faces of the pair of shutter members are pressedagainst with each other, the pair of shutter members can be contacted with each other reliably and strongly by rotating the middle shell. Therefore, the disk compartment can be prevented from being smudged by very small dusts and the like by reducing the space between the pair of shutter members and the cartridge housing. Hence, the dustproof property of the cartridge housing can be improved. Moreover, the disk cartridge can be reduced in thickness and improved in space factor so that the whole of the disk cartridge can be made compact and thin. 
     According to the disk cartridge of the present invention, since the pair of operational convexs are moved along the pair of guide grooves based on the rotation of the middle shell, the pair of shutter members can be rotated smoothly and reliably. 
     Further, according to the disk cartridge of the present invention, since the shutter open and close mechanism presses the pair of shutter members, the two shutter members can be closely contacted with each other so that the dustproof property of the cartridge housing can be improved. 
     Furthermore, according to the disk cartridge of the present invention, since the pair of shutter members are comprised of a combination of two substantially semicircular same plate materials and the pair of shutter members are symmetrically disposed across the opening portion, the pair of shutter members can be operated with a high reliability. Therefore, the opening portion can be opened and closed smoothly and reliably, and the whole of the disk cartridge can be made compact and thin. 
     Having described preferred embodiments of the invention with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments and that various changes and modifications could be effected therein by one skilled in the art without departing from the spirit or scope of the invention as defined in the appended claims.