PURPOSE: To perform manufacturing at a high yield by performing the fixing of a first jacket, wherein grooves are formed at the peripheral parts and through holes are provided, and a second jacket, wherein projections are brought into contact with the holes on the main surface, at the outside of the jackets.

CONSTITUTION: Holes 21, which are penetrated into a first jacket 11, are provided at the four corners of the first jacket 11. Grooves 23 and 25 are provided in both main surfaces at the peripheral parts of the holes 21. Projections 121 of a second jacket 111 are coupled with the holes 21 of the first jacket 11. Coupling regions 123 of the projections 121 protruding from the grooves 25 are fixed to the first jacket 11 by thermal welding from the outside of the first jacket 11. Since a magnetic disk cartridge 1 is constituted in this way, the jackets 11 and 111 can be fixed at the outer surfaces of the jackets 11 and 111. Therefore, it is not necessary to fix the jackets 11 and 111 in the insides of the jackets 11 and 111 by thermal welding and the like. Generation of dust and the like in the inside is prevented.