Abstract: An example of a relay circuit includes: a capacitor connecting both ends of a load circuit; first and second main relays disposed in power supply lines between a direct-current power supply and the load circuit; a series circuit configured by a first resistor and a precharge relay disposed in parallel with the first main relay; and a second resistor connecting both ends of the load circuit. A discharge process is performed in which both the first main relay and the precharge relay are turned on, the second main relay is turned off, and a reactive current is caused to flow through the load circuit. In this discharge process, an abnormality of the first resistor is detected based on a both-end voltage of the capacitor detected by a voltage sensor and a resistance value that is an equivalent representation of the discharge process.