Bicycle electric power unit

In the power supply device on a bicycle, as far as possible to suppress the deterioration, the power supply device (11) due to over-discharge power supply, electric power is supplied to the apparatus may be mounted on a bicycle motor unit (30), a power supply (35), power supply a control unit (36), power supply voltage detecting unit (37), a first power switch (40), a second power switch (41). The power control unit controls power on or off, the first power supply control switch into the OFF state when the power supply voltage detecting unit detects a predetermined voltage or less. Detects the power supply voltage detecting unit (35) voltage. A first power switch, when the ON state is supplied to the power control unit (36) to cut off power supply to the power supply control unit is in the off state, the power control unit in the ON state (36) is controlled to the OFF state . The second power switch (41), disposed between a power source (35) and the motor unit (30) performs on-off control by the power control unit (36).