In a charging device, a driving power supply circuit for driving a control circuit is formed as a separate system from a first power supply circuit, regardless of whether the first power supply circuit is in a driving state or a non-driving state. A microcomputer alternately and intermittently controls the first power supply circuit in the driving state or the non-driving state when the battery pack is not mounted in the charging device in order to reduce the potential difference between the battery pack and the output voltage of the first power supply circuit, thereby avoiding an excessive discharge current from the battery pack when the battery pack is mounted in the charging device.