To realize a stable operation and the suppression of heat generation in the case where an excitation current supplied from a converter is used to generate a power source voltage, a standard signal generator is provided. The standard signal generator includes an input circuit that includes: a rectification circuit that rectifies an excitation current; a resistor that is provided between a power source voltage output terminal for supplying a power source voltage and an output terminal of the rectification circuit; an amplification circuit that outputs an output voltage obtained by amplifying a voltage across both ends of the resistor; and a constant voltage circuit that performs control such that the power source voltage is constant. A power source voltage switch switches the power source voltage with the constant voltage circuit being controlled in accordance with the result of a comparison between the output voltage and a prescribed threshold value.