PURPOSE: To judge total power consumption of a battery to enable proper replacement of the battery by integrating operation time measured by a timer means to operation time read from a memory before an active state is completed.

CONSTITUTION: A CPU 18 is equipped with a timer means for counting operation time and an integrating means for integrating the operation time counted by the timer means and operation time read from a memory 20. The CPU 18 is switched to an active state by an operation start signal from a read and write device 12, and the timer means starts counting. Then when the CPU 18 ends execution of a data read instruction, it reads operation time up to the present time from the memory 20 and integrates the operation time counted by the timer means to it. This value is written into the memory 20, and the CPU 18 is switched to an inactive state. By reading the integrated value from the memory 20 with power consumption per unit hour, total power consumption is determined so that proper replacement of a battery is possible.