PURPOSE: To efficiently, safely charge a battery even when a change with passage of time is generated.

CONSTITUTION: Charging current is periodically, intermittently supplied to a battery 2. The supply amount of charging current to the battery 2 is controlled with a CPU 4a so that charge continued voltage VCHG produced between terminals when charging current is supplied to the battery 2 is set to the voltage obtained by adding a specified value of overvoltage vη to charge stopped voltage VOFF produced between terminals when supply of charging current to the battery 2 is stopped. When charging current is decreased than a specified value in the process intermittently supplying charging current to the battery 2, or the charge stopped voltage VOFF is increased than a specified value, supply of charging current to the battery 2 is stopped.