To prevent abrasion powder produced by grinding a pressed part of a slide member of a shutter with a shutter opening pin of a recording and reproducing device from entering a cartridge case.

When a disk cartridge 1 is inserted into a cartridge holder of the recording and reproducing device, the shutter opening pin provided to the cartridge holder presses a 1st assembly leg part 53 of a slide member 5, the member 5 and a shutter 4 slide to open an opening part 3c of the cartridge case 3. The 1st assembly leg part 53 has a main-surface side guide part 55 which slides along the main surface of the upper half 3a of the cartridge case 3. The main-surface side guide part 55 has the pressed part 58 positioned outside a 1st flank 53a of the 1st assembly leg part 53 along the width of the cartridge case and the shutter opening pin of the recording and reproducing device comes into contact with the pressed part 58.