PURPOSE: To enable the taking in and out of a recording medium such as a sheet like photosensitive material without damaging shading properties by providing an open mechanism coming into contact with the projection of a photosensitive material treatment device when a housing is mounted on the treatment device to separate a second plate member from a first plate member against an energizing means by a predetermined distance.

CONSTITUTION: When a magazine is separated from a laser photocomposer main body, a guide plate 2 is energized toward a guide plate 1 by a spring and felts 3, 4 are brought to a pressure contact state to become a perfect closed state. Next, a pin 27 is provided to the bottom plate 6 of a magazine housing part on the side of the laser photocomposer main body and comes into contact with the lower part of the slide pin 18 provided to the bottom part of the magazine when the magazine is mounted on the laser photocomposer main body. The slide pin 18 is pushed upwardly in such a state the magazine 39 is accurately mounted and a link member 19 is revolved and an engaging pin 25 is pushed to move a holding plate 7 and the guide plate 2 in the direction shown by an arrow A and a gap is generated between the felts 3, 4.