a preferred embodiment of the present invention will be explained in detail referring to the attached drawings . fig1 is a schematic diagram showing the structure of an image forming unit 10 of an image forming apparatus such as a color printer and the like in the embodiment of the present invention . around a photosensitive drum 11 that is an image carrier of the image forming unit 10 , there are provided a charger 12 to uniformly charge the photosensitive drum 11 sequentially with the rotation of the photosensitive drum 11 in the arrow direction s , a laser writing unit 13 that is a latent image forming unit for forming latent images on the charged photosensitive drum 11 , first through third developing devices 16 - 18 containing three color toners ; yellow ( y ), magenta ( m ) and cyan ( c ), a revolver type developing unit 20 that rotates in the arrow direction t centering around a shaft 20 b , an intermediate transfer device 21 equipped with an intermediate transfer belt 21 a that rotates in the arrow direction u , a cleaner 22 , and a charge elimination lamp 23 . below the image forming unit 10 , there are a paper supply cassette 24 for housing recording paper ( not illustrated ), an aligning roller pair 27 to synchronize the leading edge of a recording paper to the leading edge of a toner image transferred on the intermediate transfer belt 21 a along a conveying path 26 of recording paper taken out of the paper supply cassette 24 , a secondary transfer roller pair 28 for transferring a toner image secondarily on the intermediate transfer belt 21 a on a recording paper , and a fixing roller pair 30 for fixing a toner image on a recording paper . the laser wiring unit 13 forms an electrostatic latent image on the photosensitive drum 11 by applying a laser beam corresponding to a writing signal of each color according to image information input from an external computer terminal , etc . the first through third developing devices 16 - 18 have first through third developing rollers 16 a - 18 a and conveying augers 16 c - 18 c for supplying yellow ( y ), magenta ( m ) and cyan ( c ) toners in developer containers 16 b - 18 b to developing rollers 16 a - 18 a , respectively as shown in fig2 . the developing unit 20 is rotated in the arrow direction t when a stepping motor 41 drive a revolver following gear 40 provided to a revolver 31 that is a holding unit . the developing unit 20 holds the first - third developing devices 16 - 18 by installing both ends of shafts ( not illustrated ) of developing rollers 16 a - 18 a of the first through third developing devices 16 - 18 to a positioning unit 31 a that is formed on the revolver 31 through bearings . as shown in fig5 a developer fixing member 32 is mounted rotatably to the positioning unit 31 a of the revolver 31 and when the developing devices 16 - 18 are removed , the positioning unit 31 a is opened by removing a screw 32 a and rotating the developing unit fixing member 32 in the arrow direction w . further , when the developing devices 16 - 18 are mounted , after rotating the developer fixing member 32 in the reverse direction to the arrow direction w , the developing devices 16 - 18 are fixed to the revolver 31 by fixing the positioning unit 31 a with a screw . as shown in fig6 a guide 33 is formed on the revolver 31 . this guide 33 is a developing device installing guide that is a guide member to guide the developing devices 16 - 18 to a main body guide 10 b provided to a frame 10 a of the image forming apparatus by sliding them in the direction vertical to the shaft 20 b of the developing unit 20 when the developing devices 16 - 18 are attached or detached . next , the locking mechanism ( or stopping member ) and the backstop member will be described in detail . the stopping unit 37 has a stopping lever 37 b that is a supporting member attached rotatably to a fixing member 36 at a supporting shaft 37 a as shown in fig7 . the stopping lever 37 b supports a stopping hook 37 d made of polyoxy methylene ( pom ) that is a stopping member at a supporting shaft 37 c fixed on the stopping lever 37 b that is the supporting member . when the stopping hook 37 d is at the stopping position by the rotation of the stopping lever 37 b , it stops a stop claw 31 b that is a stopped portion formed as a projection or groove on the revolver 31 to stop the revolver 31 at an exchanging position for exchanging the developing devices 16 a - 18 a . further , the stopping hook 37 d is pushed by the stop claw 31 b and bent elastically about 5 ° in a reverse direction to the arrow direction x . the stopping lever 37 b has been compressed to the unstopping position shown in fig2 by a toggle spring 36 a attached to the fixing member 36 shown in fig8 . when the stopping lever 37 b is rotated in the arrow direction q , it has been compressed in the arrow direction q by the toggle spring 36 a . the stationary side 39 a of the toggle spring 36 a is attached to the fixing member 36 and the moving side 39 b is attached to the stopping lever 37 b . the toggle spring 36 a has actions to compress the stopping lever 37 b in the arrow direction q when its moving side 39 a moves to above a line x that is connecting the supporting point of the stopping lever 37 b and the stationary side 39 a of the toggle spring 36 a and compresses the stopping lever 37 b in the arrow direction r when the moving side 39 b moves to below the line x . accordingly , when the stopping lever 37 b is turned to the arrow q side as shown by the dotted line , the toggle spring 36 a compresses the stopping lever 37 b to the state turned to the arrow q side . on the other hand , when the stopping lever 37 b is turned to the arrow direction r side as shown by the one - dot chain line , the toggle spring 36 a compresses the stopping lever 37 b to the state turned to the arrow r side . further , a backstop gear 38 that is a backstop member having a built - in one - way clutch 38 a is mounted to the support shaft 37 c of the stopping lever 37 b . as a result , the backstop gear 38 cannot turn in the arrow direction y shown in fig7 . further , the support shaft 37 c coaxially supports the stopping hook 37 d and the backstop gear 38 . when the stopping hook 37 d is at the stopping position by the rotation of the stopping lever 37 d , the backstop gear 38 engages with a revolver following gear 40 formed on the revolver 31 . thus , the backstop gear 38 prevents the reverse rotating direction to the arrow direction t of the developing unit 20 . further , the backstop gear 38 is in an unengaged position wherein it does not engage with the revolver following gear 40 by the rotation of the stopping hook 37 d when the stopping hook 37 d is at the unstopping position . next , the color image forming process by the image forming unit 10 will be explained . in this image forming unit 10 , a full - color image is formed by superposing yellow ( y ), magenta ( m ) and cyan ( c ) toner images in this order . when the image forming process is started , a developing roller 16 a of the yellow ( y ) developing device 16 a of the developing unit 20 is arranged at the developing position opposite to the photosensitive drum 11 . when the image forming process is started , the photosensitive drum 11 rotates in the arrow direction s and is uniformly charged by the charger 12 , and the laser - writing unit 13 applies a laser beam according to a yellow image signal . thus , a yellow electrostatic latent image is formed on the photosensitive drum 11 . this yellow ( y ) toner is electrostatically supplied to the developing roller 16 a when reaches the developing unit 20 and a yellow ( y ) toner image is formed . then , when the toner image formed on the photosensitive drum 11 reaches a position to contact the intermediate transfer belt 21 that is turned in the arrow direction u , the yellow toner image is electrostatically transferred primarily on the intermediate transfer belt 21 by the transfer bias from the primary transfer roller 40 a . after the primary transfer , the photosensitive drum 11 is cleaned and the cleaner 22 removes residual toner and the charge elimination lamp 23 removes the charge on the surface . thereafter , in the similar manner as the yellow toner image forming process , the magenta and cyan toner image forming processes are repeated on the photosensitive drum 11 and further , the yellow ( y ), magenta ( m ) and cyan ( c ) toner images are intermediately transferred sequentially on the intermediate transfer belt 21 a from the photosensitive drum 11 and a color toner image with 3 color toners superposed is formed on the intermediate transfer belt 21 a . during these processes , the developing unit 20 is rotated in the arrow direction t by the stepping motor and the developing rollers 16 a - 18 a corresponding to toner image colors formed on the photosensitive drum 11 is arranged opposing to the photosensitive drum 11 . the yellow ( y ), magenta ( m ) and cyan ( c ) color toner images that are superposed on the intermediate transfer belt 21 a are transferred in a lump on a recording paper that is taken out in the arrow direction v from the paper supply cassette 24 and conveyed to the secondary transfer roller pair 28 in synchronous with the color toner images . a full - color image is thus completed on a recording paper p . while the image forming process is repeated as described above , the exchange maintenance of the developing devices 16 - 18 is executed when a prescribed number of image forming sheets is reached or a prescribed time elapsed . when starting the exchange maintenance , operator should first pull out the power cord of the main body of the image forming apparatus to prevent the abnormal rotation of the developing unit 20 for a malfunction , etc . of the stepping motor 41 and make the drive of the developing unit 20 free and rotatable manually . then , operator opens the side cover of the main body of the image forming apparatus and rotates the stopping lever 37 b held at the unstopping position shown in fig2 in the arrow direction q . thus , the stopping lever 37 b released from the unstopping position is compressed by the toggle spring 36 a in the arrow direction q and is held at the stopping position shown in fig3 . at this time , the backstop gear 38 engages the revolver following gear 40 and prevents the developing unit 20 from rotating in the reverse direction to the arrow direction t in fig4 . when the developing unit 20 is manually rotated in the arrow direction t and the exchanging position for exchanging the developing devices 16 - 18 is reached under this state , the revolver stop claw 31 b runs against the stopping hook 37 d of the stopping lever 37 b and the developing unit 20 is stopped at the exchanging position for exchanging the developing devices 16 - 18 in fig3 . further , this time , the stopping hook 37 d is pushed by the stop claw 31 b and bent elastically about 5 ° in a reverse direction to the arrow direction x . then , the operator should open a developer fixing member 32 in the arrow direction w by removing a screw 32 a , pull out the developing devices 16 - 18 in the direction vertical to the shaft 20 b of the developing unit 20 from the positioning unit 31 a , slide a developing device mounting guide 33 in the arrow direction z along the developing devices mounting guide 33 shown in fig3 or 5 and take out from the main body of the image forming apparatus along the main body guide 10 b . then , the operator loads the main body of the image forming apparatus with new developing devices 16 - 18 along the main body guide 10 b and the developing device mounting guide 33 , and after mounting the shafts of the developing rollers 16 a - 18 a to the positioning unit 31 a of the revolver 31 , fix the developing device fixing member 32 to the positioning unit 31 a with a screw and fix it to the revolver 31 . then , the operator rotates the stopping lever 37 b once in the reverse direction to the arrow direction q and after removing the stopping hook 37 d from the revolver stop claw 31 b . at this time the stopping hook 37 d is free and reforms its shape from the bent state . and then the operator returns the stopping lever 37 b to the stopping position by rotating it in the arrow direction q again . therefore , even when returned to the stopping position , the stopping hook 37 d does not engage with the revolver - stop claw 31 b and the developing unit 20 can be turned manually . when the developing unit 20 is rotated manually in the arrow direction t under this state , the stopping hook 37 d stops a next revolver stop claw 31 b and the developing unit 20 is stopped at a next exchanging position for exchanging next developing devices 16 - 18 . hereafter , in the similar manner as described above , the operator exchanges next color developing devices 16 - 18 . while exchanging the developing devices 16 - 18 , the developing unit 20 is firmly stopped at the exchanging position by the stopping hook 37 d attached to the stopping lever shaft 37 c fixed on the stopping lever 37 b and the backstop gear 38 attached to the stopping lever shaft 37 c . that is , the rotation of the developing unit 20 in the arrow direction t is blocked by the stopping hook 37 d that stops the stop claw 31 b formed on the revolver 31 . on the other hand , the backstop gear 38 that is engaging with the revolver following gear 40 blocks the rotation of the developing unit 20 in the reverse direction to the arrow direction t . furthermore , when the developing unit 20 tries to rotate in the direction reverse to the arrow direction t , a force to inroad into the revolver following gear 40 side acts on the backstop gear 38 for its effect of arrangement and despite of the back stop gear 38 being compressed only in the arrow direction q by the toggle spring 36 a , the reverse rotation of the developing unit 20 can be surely prevented without the possibility of disengagement of the backstop gear 38 and the revolver following gear 40 . when the developing devices 16 - 18 in three colors of yellow ( y ), magenta ( m ) and cyan ( c ) are thus exchanged completely , the image forming process can be executed when the stopping lever 37 b is depressed in the direction reverse to the arrow direction q , the stopping lever 37 b is held at the unstopping position by the toggle spring 36 a , the side cover is closed , a power cord of the main body of the image forming apparatus is connected , and the power is turned on . according to the above - mentioned embodiment , when the developing unit 20 is manually rotated in the state with the stopping lever 37 b rotated and the stopping hook 37 d moved to the stopping position for exchanging the developing devices 16 - 18 , the reverse rotation of the developing unit 20 is prevented easily and surely and the developing unit 20 can be stopped at the exchanging position for exchanging the developing devices 16 - 18 surely . accordingly , in the maintenance for exchanging the developing devices 16 - 18 , the operator &# 39 ; s safety is secured and the damage of the developing devices or their peripheral equipment caused by the reverse rotation of the developing unit 20 can be prevented . further , the power source of the stepping motor 41 is shut off in the exchange maintenance of the developing devices 16 - 18 . therefore , the developing unit 20 does not cause abnormal rotation for the malfunction of the stepping motor and further safety is obtained . furthermore , there is no possibility for deterioration of the one - way clutch 38 a provided to the backstop gear 38 for wear , its clutching accuracy can be maintained satisfactorily and the reverse rotation of the developing unit 20 can be prevented at a high accuracy . further , as the developing device mounting guide 33 is formed on the revolver 31 , the old developing devices 16 - 18 until reaching the main body guide 10 b can be easily taken out and the contamination of the peripheral portions in taking out the developing devices can be reduced . further , the present invention is not limited to the embodiment described above and various changes may be made without departing from the spirit and scope thereof . for example , toner colors in the developing devices held in the developing unit are not limited and further , one developing device containing black toner may be added and four developing devices may be provided . in addition , the image forming apparatus can be in any construction and for example , a developing device containing black toner may be arranged around the image carrier separately from the developing unit . as described above in detail , according to the present invention , the developing unit can be stopped at the exchanging position for exchanging the developing devices easily and certainly by manually rotating the developing unit after shutting off a driving source with the stopping member moved from the unstopping position to the stopping position and also the reverse rotation of the developing unit can be prevented by the backstop member that can be moved with the stopping member . accordingly , in the exchange maintenance of the developing devices , the operator &# 39 ; safety can be secured and damage of the developing devices or peripheral equipment can be prevented certainly . in addition , the further safety is assured without causing the abnormal rotation of the developing unit for the erroneous operation of the driving source .