To quickly recover function of a battery pack when it is dysfunctioned because of low temperature.

The battery unit 14 has a charging battery part 141 charged by a lithium secondary battery or the like, a plurality of temperature sensors 142 detecting temperature of the charging battery part 141, and a plurality of heaters 143 heating the charging battery part 141. The temperature of the charging battery part is detected by the temperature sensors 142 at start-up of the bicycle with electromotive assist, and when the temperature is not higher than the prescribed temperature (for example, 0°C or below), the heating of the charging battery part is started by supplying a counter electromotive force obtained from a driving motor 51 of a motor unit 50 to the heater. The temperature of the charging battery part is monitored at regular intervals, and when the temperature goes beyond the prescribed temperature (for example, 20°C or higher), the heating is stopped, and the counter electromotive force obtained from the driving motor 51 of the motor unit 50 is supplied to the charging battery part as a regenerated power.