To accurately detect the change of battery voltage without being subjected to the disturbance of noise regardless of the magnitude of a charge current by setting the time constant at the time of a voltage detecting means detecting the change of the battery voltage, according to the magnitude of a charge current.

A charger is equipped with a power circuit 11, a charge control switch 12, a detecting circuit 13 which detects the voltage of a secondary battery, and a charge control circuit 14. This is provided with a feedback circuit 102 which fixes the voltage and current from a step-down transformer 110, and is equipped with a switching power source 105, and a charge current changeover switch 104. A charge control circuit 14 terminates the charge by turning off the charge control switch 12 when the voltage of the secondary battery B detected by the voltage detecting circuit 13 amounts to a specified value. The charge control circuit 14 properly sets the time constant to detect the voltage change of the secondary battery B, according to the magnitude of the charge current being set by operating the charge current changeover switch 104. Hereby, even if the charge current is small, the change of the battery voltage is obtained surely without being disturbed by noise.