In order to provide a shutter open/close mechanism (26) suitable for use with a cartridge body (6) whose front end is formed in an arbitrary shape for easily identifying the correct direction of insertion in a recorder/player, a guide recess (36) to support a shutter plate (25) movably is formed on a main side of the cartridge body (6) to be oblique relative to the width of the cartridge body (6). The shutter open/close mechanism (26) includes a guide member (31) supporting the shutter plate (25) and movably engaged in the guide recess (36), an operating member (32) to move the guide member (31), a transmission member (33) connecting the guide member (31) and operating member (32) to each other to transmit an operating force from the operating member (32) to the guide member (31), and support surfaces (37) formed on the cartridge body (6) to support the operating member (32) movably. The shutter open/close mechanism (26) further includes a shutter locking mechanism (38) disposed at the lateral side of the cartridge body (6) to lock the shutter plate (25) against movement.