To always keep the capacity of a secondary battery so as to secure a sufficient backup time while suppressing power consumption in a power saving state.

In the power-saving state, a power-saving control part 20 waits the passing of a prescribed re-charging period after rushing into the power- saving state, temporarily release the power-saving state to make a charging part 22d charge the secondary battery 22c over a prescribed charging period. When, the charging time has passed, the part 20 instructs a CPU 11 to rush into the power-saving state to set into the power-saving state.