To provide a motorized power steering device enabled to supply main power source with power from an auxiliary power source.

An EPS 1 includes: an electric motor 21 for generating assist torque on the basis of steering torque; an auxiliary power supply device 40 having a capacitor 45, connected to a main power source 4, capable of discharging to the electric motor 21; and a control device 30 for controlling operation of the auxiliary power supply device 40. As a connection mode for the main power source 4 and auxiliary power supply device 40, the control device 30 switches over among: a second connection mode (a first supply mode) of serially interconnecting the main power source 4 and capacitor 45 whose high-voltage side is connected to the electric motor 21; first and third connection modes (a second supply mode) of serially interconnecting the main power source 4 and capacitor 45 whose low-voltage side is connected to the electric motor 21; and a fourth connection mode (a main power source charging mode) of parallel interconnecting the main power source 4 and capacitor 45.