To eliminate the necessity of managing the electric power of an in-vehicle battery, and to suppress system cost by using an auxiliary power supply properly in addition to an in-vehicle battery.

A motor controller 12 includes: the in-vehicle battery 16 which supplies electric power to an electric motor 14 of an electric power steering device 11 of a vehicle; the auxiliary power supply 19 capable of supplying electric power to the motor 14; and a control circuit 23 which charges and discharges the auxiliary power supply 19 using a predetermined current value supplied from the in-vehicle battery 16 to the motor 14 as a threshold. The charging and discharging threshold value is set to eliminate the necessity of managing the electric power for suppressing the power consumption of the in-vehicle battery 16 to the motor 14.