The power circuit device for vehicles is provided with a grounding portion for grounding portions closer to a power consumption device side than a relay of main conductive routes when the relays used for the main conductive routes extending from an accumulator of a vehicle to a fuel heating device, a catalyst heating device, an electric pump, or the like is turned OFF, so that a false power application is not executed even a short-circuit occurs in a part of a power circuit. Also, when two relays are arranged in series, the grounding portion is provided for the relay closer to the power consumption device in order to monitor a voltage level of the main conductive route and make it easy to detect a short-circuit or a disconnection occurrence.