PURPOSE: To prevent the deformation of a lock, by the constitution that the latch between a lock of a rid plate and a latch of a jacket is released with a nail of a canceler and the return of the nail is stopped on half way with the lock of the rid plate.

CONSTITUTION: When a jacket 5 is loaded to a reproducer with being latched with a rid plate 6, the latch between a lock 9 of the rid plate 6 and a latch 12 of the jacket 5 is released with a ratchet mechanism consisting of a projection 37 of a jacket guide and a canceler sublever 51 and the jacket 5 is taken out this side B. Although a canceler 46 returns to the initial position with a spring force when a projection 37 and a wing piece 53 become independent on the way of operation, a nail 48 is latched to a slit 8 of the lock 9 to restore the lock 9 to the original position. Thus, the deformation of the lock can be prevented and the normal relation between the jacket 5 and the rid plate 6 can be kept.