To measure a frequency response of a mechanism without generating a positional shift or torque saturation, and to simultaneously execute the measurement of a frequency characteristic and identification of inertia.

In this motor controller, a detector 20 detects a position of a motor 18, and a difference unit 26 calculates a motor speed from the motor position. A command generation part 11 fixes a position command to a present position. A position control part 12 generates a speed command such that the position command coincides with the motor position. A speed control part 13 generates a torque command such that the speed command coincides with the motor speed. An adder 16 adds a high-speed sweep wave from a disturbance signal generation part 15 to the torque command from a torque filter part 14 as torque disturbance to generate a new torque signal. A current control part 17 converts the torque command into a current command to drive the motor 18. A frequency response measurement part 21 is inputted with the torque command of the adder 16 and the motor speed calculated by the difference unit 26 and measures the frequency response.