A vehicular power source controller is designed so that when there is abnormal action in a microprocessor and an abnormal fuse state cannot be determined from the current of a load detected by a current sensor circuit, power supply to the load cannot be turned on and off by the operation of a combination switch corresponding to the load, but can be turned on and off by the operation of an ignition switch. It is thereby possible to prevent continuing power supply to the load when the combination switch is turned on during the abnormal action of the microprocessor, and battery exhaustion can be suppressed. Power supply to the load cannot be turned on and off by the operation of the combination switch during the abnormal action of the microprocessor, and an operator is easily made aware of the abnormal action of the microprocessor.