In a vehicular power supply system according to an embodiment, passage current of a power MOSFET of an IPD of a sub power distribution box monitored by a sub controller of the sub power distribution box is transmitted from the sub controller to a main controller of a main power distribution box. When energy storage amount of a load-based path calculated by integrating the passage current received by the main controller exceeds a predetermined value which is a overcurrent state, the main controller determines that the load-based path is in the overcurrent, and notifies the fact from the main controller to the sub controller. The sub controller switches the power MOSFET of the IPD of the load-based path set to the overcurrent by the notification to a non-conduction state to interrupt the energization of the load-based path.