To conduct the charging operation, without making a circuit structure complicated and deterioration of the charging efficiency, through prevention of continuous flow of a current in the primary side of the DC-DC converter during the charging operation, even if a battery is exhausted.

This charger comprise s the DC-DC converter for stepping up a battery voltage, a capacitor charged with the DC-DC converter, a battery check means for detecting the battery voltage conditions before the start of charging of the capacitor, a detection means for detecting that the primary current of the DC-DC converter reaches the prescribed vale and a control means for controlling the DC-DC converter. The control means changed over the condition to control the DC-DC converter, based on a detection signal from the detecting means and the condition to control the DC-DC converter with a signal, having the fixed pulse width depending on the battery condition detected with the described battery check.