A power supply device includes a boost convertor configured to perform a boost-up operation to boost a voltage supplied from an input terminal, a connection-assist diode connected in parallel to the boost convertor between the input terminal and an output terminal, and a switching element connected in parallel to the boost convertor and the connection-assist diode between the input terminal and the output terminal. A controller instructs the switching element to open and instructs the boost convertor to perform a boost-up operation for boosting the voltage at the input terminal during a boost-up period. The controller instructs the boost convertor to stop the boost-up operation when the boost-up period elapses. The controller then instructs the switching element to close the switching element. The controller then determines a state of the switching element based on a voltage at the output terminal detected. The in-vehicle power supply device can stably and accurately determine whether the switching element operates normally or not.