Motion controller and system identifying method

A motion controller accurately identifies system constants, such as inertia, viscous friction coefficient, and constant disturbance, without requiring operation, even in cases where viscous friction and/or constant disturbance are present. An identifier includes a time-differentiator for calculating an acceleration detection value afb by time-differentiating a speed detection value Vfb, a first filter for calculating Fafb by filtering the acceleration detection value afb, a second filter for calculating Fvfb by filtering the speed detection value Vfb, a third filter for calculating Ftref by filtering a torque command value Tref, and a JDC estimator for calculating an inertia identification value J_hat, a viscous friction coefficient identification value D_hat, and a constant disturbance identification value C_hat of a control object by performing time-differentiation and four arithmetic operations based on Ftref, Fvfb and Fafb.