A charging circuit includes a generator, a step-down circuit for reducing an output voltage of the generator to a predetermined voltage, a current controller for controlling a charging current that is supplied from the step-down circuit to a secondary battery, based on an amount of power generated by the generator, a mode selector for selecting charging modes, and a monitor for monitoring an amount of stored electricity. The step-down circuit receives an output of the generator, in charging, at a high voltage and a small current and is used to charge the battery at a low voltage and a large current.