Aspects described herein relate to a cap for a toner cartridge. In one example, the cap may be configured to close a toner supply opening of the toner cartridge. According to one or more aspects, the cap may include a sealing or covering portion configured to cover the toner supply opening and a shaft portion for rotatably supporting a to-be-detected rotary member. The to-be-detected rotary member 56 is rotatably supported around and fitted onto the shaft portion. Therefore, even if a toner supply opening of a cartridge is provided in a sidewall of the housing on a side where the to-be-detected rotary member is provided, e.g., a left sidewall, the toner supply opening and the to-be-detected rotary member can be provided in such a manner as to overlap each other.