An in-vehicle power supply apparatus for supplying operating power to a microcomputer is configured such that the microcomputer can securely enter into a standby state in a case where a voltage supplied from an in-vehicle power source is interrupted. The in-vehicle power supply apparatus according to the invention generates a second voltage to be supplied to a control IC and the like other than the microcomputer and a third voltage to be supplied to the microcomputer, and, in a case where the second voltage is reduced, the in-vehicle power supply apparatus transmits a reset signal and bypasses the second voltage to an input of the third voltage.