An in-vehicle power supply device (10 10a), comprising: an input terminal; an output terminal; a power supply circuit unit including a boost converter connected between the input terminal and the output terminal, and configured to perform a boosting operation to boost a voltage supplied from the input terminal, and output the boosted voltage through the output terminal, a connection-assist diode connected in parallel to the boost converter between the input terminal and the output terminal, and a switch element connected to the boost converter; and a controller configured to control the boost converter and the switch element, characterized in that said switch element is connected in parallel to the boost converter and the connection assist diode between the input terminal and the output terminal; and that the controller is configured: to detect a voltage output from the output terminal as a first voltage while instructing the switch element to open and instructing the boost converter to stop an operation of the boost converter; to, after detecting the first voltage of the output terminal, detect a voltage output from the output terminal as a second voltage while instructing the switch element to continuously open and instructing the boost converter to perform the boosting operation; and to judge, based on a difference (Vdp) between the second voltage and the first voltage, whether the boost converter is in a normal state or in an abnormal state, by comparison of the difference (Vdp) with a predetermined threshold (Vth), the difference (Vdp) being smaller than the predetermined threshold (Vth) indicating the abnormal state and the difference (Vdp) being not smaller than the predetermined threshold (Vth) indicating the normal state. an input terminal; an output terminal; a power supply circuit unit including a boost converter connected between the input terminal and the output terminal, and configured to perform a boosting operation to boost a voltage supplied from the input terminal, and output the boosted voltage through the output terminal, a connection-assist diode connected in parallel to the boost converter between the input terminal and the output terminal, and a switch element connected to the boost converter; and a boost converter connected between the input terminal and the output terminal, and configured to perform a boosting operation to boost a voltage supplied from the input terminal, and output the boosted voltage through the output terminal, a connection-assist diode connected in parallel to the boost converter between the input terminal and the output terminal, and a switch element connected to the boost converter; and a controller configured to control the boost converter and the switch element, characterized in that said switch element is connected in parallel to the boost converter and the connection assist diode between the input terminal and the output terminal; and that the controller is configured: to detect a voltage output from the output terminal as a first voltage while instructing the switch element to open and instructing the boost converter to stop an operation of the boost converter; to, after detecting the first voltage of the output terminal, detect a voltage output from the output terminal as a second voltage while instructing the switch element to continuously open and instructing the boost converter to perform the boosting operation; and to judge, based on a difference (Vdp) between the second voltage and the first voltage, whether the boost converter is in a normal state or in an abnormal state, by comparison of the difference (Vdp) with a predetermined threshold (Vth), the difference (Vdp) being smaller than the predetermined threshold (Vth) indicating the abnormal state and the difference (Vdp) being not smaller than the predetermined threshold (Vth) indicating the normal state. to detect a voltage output from the output terminal as a first voltage while instructing the switch element to open and instructing the boost converter to stop an operation of the boost converter; to, after detecting the first voltage of the output terminal, detect a voltage output from the output terminal as a second voltage while instructing the switch element to continuously open and instructing the boost converter to perform the boosting operation; and to judge, based on a difference (Vdp) between the second voltage and the first voltage, whether the boost converter is in a normal state or in an abnormal state, by comparison of the difference (Vdp) with a predetermined threshold (Vth), the difference (Vdp) being smaller than the predetermined threshold (Vth) indicating the abnormal state and the difference (Vdp) being not smaller than the predetermined threshold (Vth) indicating the normal state.