A recording medium cartridge including a case with two main surfaces which are disposed substantially parallel to each other and side surfaces which connect the main surfaces. An opening is provided on one of the side surfaces which is disposed toward a direction in which the recording medium cartridge is loaded into the drive device. A through hole is provided on one of the main surfaces. A cover member opens and closes the opening and is urged in a closing direction in which the cover member closes the opening. A cam member is mounted on the cover member. When the recording medium cartridge is loaded into the drive device, an engagement pin disposed at the drive device passes through the through hole to move the cover member in an opening direction.