PURPOSE: To facilitate the opening of the cap part of a cassette body and to simplify the reading of image information by providing an energizing means which energizes the cap part and holds the same in the open position where the entire surface of the accumulative type fluorescent material sheet in the cassette body to said body.

CONSTITUTION: The cap part 3 is energized by a pair of springs 4 which are the energizing means fixed at one end to the cassette body 2 and at the other end to the cap part 3. A locking means 5 which holds the cap part 3 to the closed position by engaging with the cap part 3 is mounted to the front face of the cassette body 2. The locking means 5 is energized by a spring 6 to the position where said means projected toward the inside of the cassette 2, by which said means is engaged with the cap part 3 to hold the cap part in the closed position by overcoming the energizing force of the springs 4. The cap part 3 is opened simply by pulling the locking means 5 according to this constitu tion. Reading is, therefore, executable without removing the accumulative type fluorescent material 1 from the cassette body 2.