A power conversion device configured to be mounted on a vehicle, the power conversion device comprising: a power converter (11, 12) configured to convert power supplied to the vehicle; a relay (21, 22, 23, 24, 25) provided in a power supply line connected to the power converter; and a welding detector configured to detect welding of the relay; wherein the welding detector includes: a first resistor (R301, R31, R32, R33) connected to a terminal of the relay on a first side; characterised in that the welding detector further includes: a capacitor (C101, C201, C11, C21, C41, C51, C61, C71, C81, C91) and a second resistor (R101, R201, R11, R21, R41, R51, R61, R71, R81, R91), both of which are connected to a terminal of the relay on a second side; an application unit configured to apply an inspection signal to the relay via the capacitor and the second resistor; and a determiner connected between the capacitor and the second resistor, the determiner configured to detect a signal based on application of the inspection signal by the application unit to determine whether or not the relay is welded; and wherein the relay includes, on the second side, a first terminal and a second terminal, and the relay is configured to switch a terminal connected to the terminal on the first side between the first terminal and the second terminal. a power converter (11, 12) configured to convert power supplied to the vehicle; a relay (21, 22, 23, 24, 25) provided in a power supply line connected to the power converter; and a welding detector configured to detect welding of the relay; wherein the welding detector includes: a first resistor (R301, R31, R32, R33) connected to a terminal of the relay on a first side; characterised in that the welding detector further includes: a capacitor (C101, C201, C11, C21, C41, C51, C61, C71, C81, C91) and a second resistor (R101, R201, R11, R21, R41, R51, R61, R71, R81, R91), both of which are connected to a terminal of the relay on a second side; an application unit configured to apply an inspection signal to the relay via the capacitor and the second resistor; and a determiner connected between the capacitor and the second resistor, the determiner configured to detect a signal based on application of the inspection signal by the application unit to determine whether or not the relay is welded; a first resistor (R301, R31, R32, R33) connected to a terminal of the relay on a first side; characterised in that the welding detector further includes: a capacitor (C101, C201, C11, C21, C41, C51, C61, C71, C81, C91) and a second resistor (R101, R201, R11, R21, R41, R51, R61, R71, R81, R91), both of which are connected to a terminal of the relay on a second side; an application unit configured to apply an inspection signal to the relay via the capacitor and the second resistor; and a determiner connected between the capacitor and the second resistor, the determiner configured to detect a signal based on application of the inspection signal by the application unit to determine whether or not the relay is welded; and wherein the relay includes, on the second side, a first terminal and a second terminal, and the relay is configured to switch a terminal connected to the terminal on the first side between the first terminal and the second terminal.