To save the use amount of a device in a non-volatile memory equal to or lower grade than the conventional one with smaller error than before, without exceeding the write frequency limit of the non-volatile memory, without the need of configuration of a device for detecting the off state of a power supply, and without providing a write frequency counter in the non-volatile memory.

When the life of a development unit 4 is 20,000,000 revolutions, and the write enable frequency of the non-volatile memory is 200,000 revolutions, two seconds before, the value of a RAM counter 30 showing a remainder in the preceding calculation and the value of a RAM counter 20 showing this time rotational frequency are added, and the value is divided by a threshold = 200, its remainder is saved in the RAM counter 30, and the double of the quotient is added to the write frequency counter. When the value of the write frequency counter is one or more, the write frequency counter is subtracted by one, the value to which the value of the RAM counter 20 is added is written in the non-volatile memory, and the RAM counter 20 is cleared.