A first voltage detector detects whether the voltage of a secondary cell has reached to 1.2V and a first timer counts a time duration for which the voltage is equal to or above 1.2V. If the time counted by the first timer reaches one hour, a second timer starts counting time for one hour. An amount of charge accumulated in the secondary cell is displays after counting of the one hour by the second timer is over. As a result, an accurate display of the amount of charge accumulated in the secondary cell can be achieved.