When a process cartridge is installed in a laser printer main body, a projecting rod, which is provided at a toner tank and which projects from a side surface of a housing of the cartridge, enters into a U-shaped positioning portion such that the cartridge is positioned. The projecting rod pushes a lever of a switch downward and turns the switch on. Operation of the printer becomes possible, and power is supplied to a light scanning device. Even if the cartridge, in which the toner tank is not installed, is installed in a laser printer, because the projecting rod does not project out, the process cartridge is not positioned, and the switch remains off. The printer does not operate and power is not supplied to the light scanning device.