A control circuit 6 determines whether a difference between an input/output potential difference V11 and an input/output potential difference V13 when bypass switches SW1 and SW2 are again turned on is equal to or smaller than a threshold value Vth2 in a case where the control circuit 6 determines, at normal times other than an engine restart after being idle-stopped, that a difference between the input/output potential difference V11 of a bypass circuit when the bypass switches SW1 and SW2 are turned on and an input/output potential difference V12 when the bypass switch SW1 is turned on and the bypass switch SW2 is turned off is equal to or larger than a threshold value Vth1 and that the bypass switch SW1 has an open fault, and validates the determination of the open fault of the bypass switch SW1 in a case where the control circuit 6 determines that the difference between the input/output potential differences V11 and V13 is equal to or smaller than a the threshold value Vth2.