An auxiliary power source device includes a main power source-side path, an auxiliary power source-side path, a power source control circuit, an analog determination circuit that is provided separately from the power source control circuit, and a power source switching circuit. The power source control circuit controls switching between a conduction state and an interruption state on the main power source-side path and the auxiliary power source-side path. The analog determination circuit determines an occurrence of abnormality in the main power source and generates a switching signal when the occurrence of the abnormality in the main power source is determined. The power source switching circuit switchs a power source for supplying power to a power supply target, from the main power source to the auxiliary power source.