ELECTRIC POWER STEERING SYSTEM

An auxiliary power supply device (13) includes an auxiliary power source (24) and a booster circuit (30). The auxiliary power supply device (13) is configured to be switched among a charging state, a holding state, and a discharging state. The auxiliary power source (24) is configured to be switched between a serial connection mode and a parallel connection mode. The auxiliary power source (24) is configured to perform boosting to supply power to the power supply target when the auxiliary power source (24) is switched to the serial connection mode, and perform backup when the auxiliary power source (24) is switched to the parallel connection mode.