1. A position control device that has a configuration which determines a torque command value of a servo torque by a feedback loop of position and velocity and a feedforward of velocity and acceleration and deceleration torque and controls, by a servo motor, an angle of rotation of an axis of rotation of a numerical control machine in accordance with a position command value from a precedence device, the position control device comprising: a signal vector generating unit that generates, on the basis of actual motion of the axis of rotation, a signal vector based on the angular acceleration and angle of rotation thereof; an adaptive loop calculating unit that performs adaptive identification from the signal vector and a torque command value of axis-of-rotation conversion with respect to an identification model based on a motion equation of a target plant including an axis of rotation comprising a term relating to the angular acceleration of the axis of rotation and a term relating to the angle of rotation and which calculates an estimate value of a parameter vector comprising an element pertaining to a coefficient of a term relating to angular acceleration of the identification model and an element pertaining to a coefficient of a term relating to angle of rotation; a moment-of-inertia coefficient calculating unit that calculates a moment-of-inertia coefficient on the basis of the element pertaining to the coefficient of the term relating to angular acceleration in the estimate value of the parameter vector; and a nonlinear element calculating unit that calculates nonlinear element torque compensation on the basis of the element pertaining to the coefficient of the term relating to angle of rotation in the estimate value of the parameter vector and the angle of rotation of the axis of rotation, wherein the position control device corrects the torque command value by the moment-of-inertia coefficient and the nonlinear torque compensation.