Diagnosis apparatus and diagnosis method for relay circuit

An example of a relay circuit includes: a capacitor (C) connecting both ends of a load circuit (30 and 40); first and second main relays (81 and 82) disposed in power supply lines between a direct-current power supply (50) and the load circuit (30 and 40); a series circuit configured by a first resistor (R1) and a precharge relay (83) disposed in parallel with the first main relay (81); and a second resistor (R2) connecting both ends of the load circuit (30 and 40). A discharge process is performed in which both the first main relay (81) and the precharge relay (83) are turned on, the second main relay (82) is turned off, and a reactive current is caused to flow through the load circuit (30 and 40). In this discharge process, an abnormality of the first resistor (R1) is detected based on a both-end voltage (V) of the capacitor (C) detected by a voltage sensor (70) and a resistance value (R dis ) that is an equivalent representation of the discharge process.