A cartridge is set in a printer main unit, and has a remaining toner measurer which measures toner in a toner chamber upon setting of the cartridge. A cartridge memory stores information of a remaining toner amount measured previously. CPU compares the remaining toner amount measured newly with the remaining toner amount measured previously and read from the cartridge memory. If there is an increase in the toner, the cartridge is determined as improper cartridge reloaded with the toner. An alarm signal is generated. Also, printing is inhibited. In another preferred embodiment, shortage information is assigned to the cartridge upon using up the toner. The cartridge, if it has the shortage information, is determined as improper. If a characteristic of the toner is not within a reference range, the cartridge is determined improper. In another preferred embodiment, an upper limit of the number of times of toner reloading is predetermined. The cartridge is determined improper if the reloading time number comes up to the upper limit. The improper cartridge is set in a state in which operation is inhibited. Also, when the cartridge is disassembled, the improper cartridge is inhibited from operating.