In a vehicular power supply system, a main battery supplies electric power to both main machine drive circuit, which drives a main motor for driving a vehicle, and a steering assist drive circuit, which drives a steering assist motor of a power steering apparatus. A main machine switching circuit, which is capable of shutting off power supply to the main machine drive circuit, and an auxiliary machine switching circuit, which is capable of shutting off power supply to the steering assist drive circuit are provided in parallel. As long as the auxiliary machine switching circuit is turned on, power supply to the steering assist drive circuit is continued even when the main machine switching circuit is turned off.