To provide a motor driving controller which can securely suppress a of rush current by appropriately keeping charge voltage to a capacitor and to provide a motor power steering system using the controller.

The motor driving controller includes an electric motor 13, and a motor driving circuit 110 which drive-controls the electric motor 13 based on power from a DC power supply 15. The controller also includes a power relay circuit 120 arranged between the DC power supply 15 and the motor driving circuit 110 and a precharge circuit 150 installed in parallel to the power relay circuit 120. The precharge circuit 150 includes a capacitor 151 connected between a motor driving circuit-side contact of the power relay circuit 120 and a ground, a series circuit 155 of a switching element 152 and a precharge resistor 154, which are connected in parallel to the power relay circuit 120, and a precharge driving part 101 driving the switching element 152 by a pulse width modulation signal.

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