To obtain a method for identifying the inertia in real time through a small quantity of simple operation.

In the method for identifying the inertia of motor 16 speed controller having at least proportional term and integral term, an integral term is subtracted from a torque command obtained in the speed controller and passed through a filter using the speed output of the motor 16 to obtain a value 1, and the speed output of motor is passed through a specific filter and differentiated to obtain a value 2. The value 1 is divided by the value 2 to obtain an inertia identification value including the motor and load.