PURPOSE: To prevent illegal operation of sheet counting in image formation for a printer for which fees are charged in accordance with counting on the number of sheets by a method wherein judgement is made to find whether a parameter obtained by inspection with an inspection means agrees with a specified parameter, and in the case where the judgement is negative, the printer is put in an irrevocable state of being in error.

CONSTITUTION: When an electric current is on, voltage in +5v from a power source is supplied through a diode 713, and when the power source is off, a backup voltage (approximately in 3V) is supplied to a RAM 107 from a battery 710 through a total counter 81 and a diode 714. Therefore, supply of electric current to the RAM 107 is cut off if a connector is pulled out or a wiring course in the total counter 81 is broken off, and data in the RAM 107 are broken. With electric current supplied from the power source, a controller executes checking on character arrangement in the RAM 107 and in the case where a judgement is made that an illegal operation is applied to the total counter, the controller decides that the checking shows 'no good', and puts a printer in the state where copying cannot be made unless operated by a serviceman.