PURPOSE: To appropriately set a timing for releasing a lock, and to exactly position a sheet pack by releasing a lock of a discharge member of the sheet pack by a closing operation of a cover.

CONSTITUTION: A discharge plate 112 of a sheet pack 10 is attached to a bucket 26 so as to be freely slidable in the inserting and discharging directions, and energized in the discharging direction by a spring 110. Also, a lock member 122 of the discharge plate 12 is attached to a pin 120 of the bucket 26 so as to be turnable. When the sheet pack 10 is inserted into the bucket, the tip of the pack moves the discharge plate 112 to the left, and thereafter, locked by a detaining part 126 of the lock member 122. Subsequently, when a cover 28 is depressed, positioning of the pack 10 is executed, and also, the cover 28 abuts on the rear end part 130 of the lock member 122 and releases a lock, therefore, the timing becomes appropriate, and the pack is positioned exactly.