POWER SOURCE DEVICE

A power steering system includes: a main power source; an auxiliary power source connected to the main power source via a power source switch; a charge circuit connected to the main power source via the power source switch and configured to charge the auxiliary power source on the basis of the main power source; a switching circuit that switches a power supply mode; and a unit that turns off the power source switch after the auxiliary power source is charged by the charge circuit until the voltage of the auxiliary power source becomes equal to or more than a predetermined voltage in the case where the voltage of the auxiliary power source is less than the predetermined voltage when a key switch for a vehicle is turned off.