A power conversion device, comprising: a power conversion unit that converts direct current power into alternating current power between a direct current power supply and a multi-phase rotating electrical machine; a control unit that controls an operation of the power conversion unit; a step-up power supply circuit connected to the power conversion unit and supplying an operation power supply to the power conversion unit; an internal power supply circuit, connected to an anode terminal of the direct current power supply via an external power supply switch, that outputs a predetermined voltage based on a state of the external power supply switch and outputs a step-up power supply circuit start-up signal to the step-up power supply circuit; an internal power supply switch, connected between the direct current power supply and the step-up power supply circuit, that opens or closes a current path from the direct current power supply to the step-up power supply circuit based on an output voltage of the step-up power supply circuit; and a bias power supply switch, connected in parallel with the internal power supply switch, that opens or closes a current path from the direct current power supply to the step-up power supply circuit based on an output voltage of the internal power supply circuit, wherein when the output voltage of the internal power supply circuit is equal to or greater than a predetermined value at a time of a start-up by the external power supply switch, the bias power supply switch opens a current path, the step-up power supply circuit steps up and outputs a predetermined voltage based on the step-up power supply circuit start-up signal, and the internal power supply switch opens a current path, when the output voltage of the internal power supply circuit is equal to or lower than a predetermined value at a time of a stop by the external power supply switch, the step-up operation of the step-up power supply circuit is stopped in response to a stopping of the step-up power supply circuit start-up signal and the internal power supply switch closes the current path, and when the output voltage of the internal power supply circuit becomes lower than a predetermined value, the bias power supply switch closes the current path. a power conversion unit that converts direct current power into alternating current power between a direct current power supply and a multi-phase rotating electrical machine; a control unit that controls an operation of the power conversion unit; a step-up power supply circuit connected to the power conversion unit and supplying an operation power supply to the power conversion unit; an internal power supply circuit, connected to an anode terminal of the direct current power supply via an external power supply switch, that outputs a predetermined voltage based on a state of the external power supply switch and outputs a step-up power supply circuit start-up signal to the step-up power supply circuit; an internal power supply switch, connected between the direct current power supply and the step-up power supply circuit, that opens or closes a current path from the direct current power supply to the step-up power supply circuit based on an output voltage of the step-up power supply circuit; and a bias power supply switch, connected in parallel with the internal power supply switch, that opens or closes a current path from the direct current power supply to the step-up power supply circuit based on an output voltage of the internal power supply circuit, wherein when the output voltage of the internal power supply circuit is equal to or greater than a predetermined value at a time of a start-up by the external power supply switch, the bias power supply switch opens a current path, the step-up power supply circuit steps up and outputs a predetermined voltage based on the step-up power supply circuit start-up signal, and the internal power supply switch opens a current path, when the output voltage of the internal power supply circuit is equal to or lower than a predetermined value at a time of a stop by the external power supply switch, the step-up operation of the step-up power supply circuit is stopped in response to a stopping of the step-up power supply circuit start-up signal and the internal power supply switch closes the current path, and when the output voltage of the internal power supply circuit becomes lower than a predetermined value, the bias power supply switch closes the current path.