Provided is an in-vehicle power supply apparatus which supplies power to both a high-power load such as an actuator, and the CPU for controlling the high-lower load, and which is capable of preventing reduction in voltage of the CPU even when voltage of the in-vehicle power supply is momentarily reduced due to power consumption of the high-power load, and which include an auxiliary power supply having its capacity reduced so as to enable reduction of the size of the power supply apparatus. The in-vehicle power supply apparatus for supplying a power to both a power load, and a CPU for controlling the power load comprises: a first distribution line for distributing, to the power load, the power supplied from an in-vehicle power supply through a bus; a second distribution line for distributing, to the CPU, the power supplied from the in-vehicle power supply through the bus; an auxiliary power supply, connected to the second distribution line, for supplying the power to the CPU when a voltage is reduced in the bus; and a backward flow prevention section for preventing a charge stored in the auxiliary power supply from flowing into the bus.