To provide a power source capable of properly diagnosing a relay according to a state of a capacitor in a short time.

A microcomputer E3 supplies a signal turning off a relay E1 and a relay E2 through a signal wire H4 and a signal wire H5. The microcomputer E3 detects voltage Vc of a capacitor C1 terminal, and determines that a charge of the capacitor C1 is empty when the voltage Vc is 0 [V] or lower than a threshold, and conducts failure diagnosis of the relays E1, E2 according to diagnosis before charge. When the voltage Vc of the capacitor C1 is higher than the threshold, the microcomputer E3 determines that charges accumulate in the capacitor C1, and conducts failure diagnosis of the relays E1, E2 according to diagnosis during charge.