1. A complex curved machining surface creation method for generating a curved machining surface with a rounded section curve having a radius R provided at an intersection where a first curve approximated by straight lines on a first three-dimensional curved machining surface intersects a second curve approximated by straight lines on a second three-dimensional curved machining surface, wherein a point of intersection of the first an second curves being denoted by P.sub.0, straight line approximation points located one after another from the point of intersection P.sub.0 to a starting point of the first curve being denoted by P.sub.j (j=0, 1, 2,. .. ), and staight line approximation points located one after another from the point of intersection P.sub.0 to an end point of the second curve being denoted by P.sub.j (j=0, 1, 2,. .. ), said method comprising the steps of: performing operations of setting P.sub.-s =P.sub.-E =P.sub.i-1, P.sub.s =P.sub.j and P.sub.E =P.sub.j+1 (where i, j each have an initial value of 0), and obtaining points of tangency P.sub.-T, P.sub.T at which two straight lines P.sub.-S P.sub.-E and P.sub.S P.sub.E are tangent to a circle of radius R, where P.sub.-S, P.sub.-E, P.sub.S and P.sub.E are end points of the straight lines; determining whether the points of tangency P.sub.-T, P.sub.T lie between P.sub.-E .about.P.sub.0 an P.sub.0 .about.P.sub.E, respectively; designating a circular arc P.sub.-T,P.sub.T the rounded section curve of the curved surface creating the complex curved machining surface joining the first and second three-dimensional curved machining surface a and inserted at the intersection if the points of tangency P.sub.-T, P.sub.T lie between P.sub.-E .about.P.sub.0 and P.sub.0 .about.P.sub.E, respectively; executing said steps of performing, determining an designating, with the operation of setting i=i-1 being performed if P.sub.-T, does not lie between P.sub.-E .about.P.sub.0 and the operation of setting j=j+1 being performed if P.sub.T does not lie between P.sub.0 .about.P.sub.E ; and machining the complex curved machining surface.