Abstract: A current supply unit supplies a current between first and second current output terminals. A voltage measurement unit measures a voltage input to first and second voltage input terminals. A control circuit causes a first switch unit to connect the first current output terminal to a first terminal of a reference resistor and to connect the second current output terminal to a second terminal of the reference resistor, causes a second switch unit to connect the first voltage input terminal to the first terminal of the reference resistor and to connect the second voltage input terminal to the second terminal of the reference resistor, and causes a voltage measurement unit to measure a voltage between the first and second voltage input terminals, thereby performing self-diagnosis of a path through which a current flows.