PURPOSE: To reduce the cost of a material, to increase production, to make the production efficiency, to improve accuracy and to enhance the degree of freedom in terms of design.

CONSTITUTION: One side part of a disk cartridge C is provided with an operation plate part 2 which is attached so as to abut on the front, the back and the side surfaces of the cartridge C, which covers the edge part of the side surface and the front and the back surfaces along the side surface, whose cross section is U-shaped and which is extended in one direction by keeping the U-shaped cross section. Besides, it is provided with opening and closing plate parts 4 and 4 which are successively provided on front and back surface operation plates 3 and 3 covering the edge part of the front and the back surfaces of the plate part 2 and which close aperture parts for reading a signal 7 and 7 formed on the front and the back surfaces of the cartridge C within the area and open them outside the outside edge parts.