To obtain a battery charging method in which the contact failure of a terminal is eliminated and in which a charger can be miniaturized.

First, a constant charging current is supplied to a battery which is mounted and attached. Then, when the supply of the charging current is started, the voltage of the battery is measured at every prescribed time. After that, the time which elapses until the measured voltage of the battery reaches a predetermined voltage is computed. Then, the computed time and a predetermined reference time are compared. After that, on the basis of a comparison result, the capacity of the battery is judged. Then, on the basis of the judged capacity of the battery, the charging current is controlled.