A disk cartridge has a write protect portion for switching between a write-allowed state and a write-inhibited state. The write protect portion is equipped with two prisms having reflection faces different in reflectivity which are rotatably supported therein. In a drive device in which the disk cartridge is loaded, an amount of light, which is irradiated to the write protect portion and then reaches a photodetecting portion of the drive device, is varied by rotating a shaft. On the basis of the amount of the light reaching the photodetecting portion, the drive device judges whether the disk cartridge is in a write-inhibited state. According to the disk cartridge, dust-proof performance is not lost, and occurrence of mis-detection due to deformation of the disk cartridge can be prevented.