PURPOSE: To make on-line inertia estimation possible even in applications in which load inertia varies during operation.

CONSTITUTION: A motor speed deviation nM from a motor speed command nM* is detected by a deviation detector 11. The output from the deviation detector 11 is input to an adder 13 through a speed control amplifier 12. The adder 13 is provided with an estimated load torque signal from a load torque estimation observer 15. The adder 13 sums up these two inputs to output a motor torque τM, which is then fed to a 2-inertia system model 14. The estimated load torque signal is fed to an inertia error estimator 16, which performs arithmetic on the signal to obtain an inertia error K-1.