In order to obtain an NC device that can easily shift by an arbitrary amount a main spindle single-revolution reference signal without being dependant on hardware and while rotating a main spindle, the NC device includes a main spindle reference angle correction amount setting unit for inputting a correction angle of a reference point, a main spindle within-single-revolution feed-back position correction unit for correcting the reference point by a correction angle set by the main spindle reference angle correction amount setting unit, and a main spindle within-single-revolution command position correction unit for correcting a main spindle within-single-revolution command position by the correction angle set by the main spindle reference angle correction amount setting unit.