A pre-processing arithmetic means (2) interprets an NC program (1) containing an interpolation command for an NURBS curve expressed by the function of a parameter (t) and calculates the distance DELTA L of movement during an interpolating period T. A parameter value variation calculating means (3a) calculates the amount of change DELTA t of the parameter (t) caused by the movement of the distance DELTA L from the present position specified by the present value t1 of the parameter (t). An amount-of-movement calculating means (3b) calculates the position when the value of the parameter (t) becomes (ti+ DELTA t) by using the defining equation of the NURBS curve. Then the means (3b) calculates the amount of movement to the position along each axis and outputs an interpolation pulse for each axis.