A secondary-control-system power supply line is provided to supply electricity from a secondary power supply apparatus to an electronic control apparatus. When a primary power supply voltage becomes equal to or lower than a primary power supply determination voltage, a power supply control section turns a step-down switching element on and off to thereby supply electricity from the secondary power supply apparatus to the electronic control apparatus. When a secondary power supply voltage becomes equal to or lower than a secondary power supply determination voltage, the power supply control section turns a secondary power supply relay off so as to stop the supply of electric power from the secondary power supply apparatus to a motor drive circuit, and continues the supply of electric power to the electronic control apparatus. Thus, resetting of a microcomputer of the electronic control apparatus, which would otherwise occur due to a drop in the power supply voltage, is prevented.