Abstract: A magnetic disk cartridge has a first cassette shell half provided with engagement holes and a second cassette shell half provided with engagement pins. Each engagement hole has an entrance portion enabling insertion of an engagement pin and a shoulder-like engagement portion following the entrance portion. Each engagement pin has an elastic structure enabling it to be compressed and is formed at its tip portion with a bulged portion. The bulged portion is formed at its distal end with a tapered portion and at its proximal end with a sloped provisional retaining surface. The first and second cassette shell halves are formed on inner surfaces thereof with abutting surfaces. The cassette shell halves are provisionally assembled in the final assembly step by bringing the sloped provisional retaining surface of the bulged portion of the engagement pin engaged with the engagement hole into engagement with the shoulder-like engagement portion and bringing the abutting surfaces into abutment with each other.