A reverse current protection circuit includes a MOSFET and an on/off controller. When the on/off controller applies an on-state control voltage to a gate of the MOSFET, a main power supply input through a main power supply line is output to a common connection node via the MOSFET; and when an off-state control voltage is applied to the gate of the MOSFET, the electrical connection between the main power supply line and the common connection node is interrupted to prevent a reverse current. A diode is connected to enable a forward current flowing from a sub power supply line to the common connection node and the reverse current is prevented. The voltage drop of the main power supply, when the on-off controller applies the on-state control to the gate of the MOSFET, is lower than the voltage drop of the sub power supply through the diode.