PURPOSE: To avoid overcharging of a battery by detecting the battery voltage under a predetermined charging current applying state upon elapse of a predetermined time after start of the charging and then correcting the second phase charging time based on a detected battery voltage.

CONSTITUTION: A controller 17 comprises an input interface 18 inputted with signals from various sensors, a MOR 19, a RAM 20, a CPU 21 performing calculations based on the input signals, programs and data, and an output interface 22 for delivering the calculation results in the form of control signals to the outside. Since the residual capacity of the battery can be determined based on the battery voltage for a constant charging current, a control program for correcting the charging time based on the battery voltage for a predetermined charging current fed upon elapse of a predetermined time after start of charging is stored in the controller 17 and the charging time is corrected based on the residual capacity of battery at the time of starting the charging. This constitution shortens the second phase charging time and prevents overcharging of the battery.