In a power supply apparatus 40, a power supply circuit is formed by a primary power supply circuit 50 which includes a high-voltage battery 51 and a step-down circuit 55 for stepping down the voltage of the high-voltage battery 51, and a secondary power supply circuit 60 which includes a low-voltage battery 61 and a step-up circuit 70 for stepping up the voltage of the low-voltage battery 61. The primary power supply circuit 50 and the secondary power supply circuit 60 are connected in parallel. An output voltage of the secondary power supply circuit 60 is set to be lower than an output voltage of the primary power supply circuit 50. When the output voltage of the primary power supply circuit 50 becomes lower than a target voltage of the secondary power supply circuit 60, the voltage stepped-up by the step-up circuit 70 is supplied to a motor drive circuit 32. Accordingly, power supply backup of an electric power steering apparatus can be performed as low cost.