A vehicular electric power source controller that controls the electrification of an electrical load mounted in a vehicle includes: a first electrical load that are a portion of the electrical load mounted in the vehicle; a second electrical load that are electrified preferentially over first electrical loads when a vehicle is parked; an electric power source-switching ECU, an ACC relay and an IG relay that switch not only the electrification of the first electrical load to but also the electrification of the second electrical load; and an electric power source management ECU, a parked state ACC relay and a parked state IG relay that switch the electrification of only the second electrical load. The vehicular electric power source controller curbs wasteful electricity consumption while operating an electrical load that is used while the vehicle is parked.