An electrical power supply system for a vehicle is provided, which can eliminate proliferation of adverse effect on other control modules caused within one of a plurality of control modules. This is accomplished by provision of the power supply system comprising: battery 10 for supplying power; power line PL in a loop lay-out; a plurality of control modules BCM 100, VCM 110, FIM 120, DDM 130, RIM 140, CCM 150 and PDM 160 for a plurality of accessory systems, which are connected via said power line PL in loop lay-out, wherein a power control module (PCM) 20 for controlling the engine is connected to the power supply source via another power line 22 different from the power line PL.