An involute interpolation method is provided for machining operations in a numerical control apparatus, in which a rotational direction of an involute curve, a center position of a base circle (C), and a radius (R) of the base circle (C) are instructed, and an interpolation is performed with respect to an involute curve having a start point (P.sub.s) on a first involute curve (IC1) and an end point (P.sub.e) on a second involute curve (IC2). According to such a method, machining operations can be done by the use of a specifically configured involute curve which is distinct from the two involute curves.