An in-vehicle power supply device comprising: an input terminal, an output terminal configured to be connected to a load, a power supply circuit including a boost convertor connected between the input terminal and the output terminal, the boost converter being configured to perform a boost-up operation in which the boost converter boosts a voltage supplied from the input terminal and outputs the boosted voltage from the output terminal, a connection-assist diode connected in parallel to the boost convertor between the input terminal and the 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 configured to detect a voltage at the output terminal and control the boost convertor and the switching element; and a start-signal receiving section connected to the controller, characterized in that, in response to a start signal which is received by the start-signal receiving section, the controller is configured to: perform a first determining operation in which the controller instructs the switching element to open the switching element and instructs the boost convertor to perform the boost-up operation during a boost-up period; perform a second determining operation in which the controller instructs the boost convertor to stop the boost-up operation at a first time point when the boost-up period elapses and instructs the switching element to close the switching element at a second time point when a predetermined period elapses from the first time point; and perform a third determining operation in which the controller determines a state of the switching element based on a voltage at the output terminal detected after terminating the second determining operation. an input terminal, an output terminal configured to be connected to a load, a power supply circuit including a boost convertor connected between the input terminal and the output terminal, the boost converter being configured to perform a boost-up operation in which the boost converter boosts a voltage supplied from the input terminal and outputs the boosted voltage from the output terminal, a connection-assist diode connected in parallel to the boost convertor between the input terminal and the 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 boost convertor connected between the input terminal and the output terminal, the boost converter being configured to perform a boost-up operation in which the boost converter boosts a voltage supplied from the input terminal and outputs the boosted voltage from the output terminal, a connection-assist diode connected in parallel to the boost convertor between the input terminal and the 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 configured to detect a voltage at the output terminal and control the boost convertor and the switching element; and a start-signal receiving section connected to the controller, characterized in that, in response to a start signal which is received by the start-signal receiving section, the controller is configured to: perform a first determining operation in which the controller instructs the switching element to open the switching element and instructs the boost convertor to perform the boost-up operation during a boost-up period; perform a second determining operation in which the controller instructs the boost convertor to stop the boost-up operation at a first time point when the boost-up period elapses and instructs the switching element to close the switching element at a second time point when a predetermined period elapses from the first time point; and perform a third determining operation in which the controller determines a state of the switching element based on a voltage at the output terminal detected after terminating the second determining operation. perform a first determining operation in which the controller instructs the switching element to open the switching element and instructs the boost convertor to perform the boost-up operation during a boost-up period; perform a second determining operation in which the controller instructs the boost convertor to stop the boost-up operation at a first time point when the boost-up period elapses and instructs the switching element to close the switching element at a second time point when a predetermined period elapses from the first time point; and perform a third determining operation in which the controller determines a state of the switching element based on a voltage at the output terminal detected after terminating the second determining operation.