To easily and inexpensively correct a clock delay during service interruption, and to also reduce labor for clock management.

In a step 114, the present time of power recovery is compared with the service interruption time, and error data are counted. A corrected value is calculated based on the error data (for example, calculated based on estimated value data) in a step 116, and the present time by the corrected value is corrected in a step 118. In other words, the clock delay during the service interruption can be easily and inexpensively corrected without increasing the number of components such as a thermistor or an AT crystal oscillator. Since time correction is performed, a user's burden of clock setting is alleviated.