When such a failure as the output from a voltage sensor (13) becomes unavailable is detected, a controller (30) controls a connecting section (40S) to disconnected state and controls a voltage converter (12S) to voltage unconverted state so that the voltage of a positive pole bus (PL2) is outputted to the terminal of the voltage converter (12S) on the connecting section (40S) side, and performs voltage control of the positive pole bus (PL2) based on the output from a voltage sensor (21S) in place of the output from the voltage sensor (13). Consequently, a power supply unit for a vehicle which can perform escape running while maintaining the traveling performance as much as possible even upon occurrence of failure in the sensor is provided.