1. A method of specifying and machining a three-dimensional curve on a workpiece using a curve generating apparatus, the method comprising: entering, into the apparatus, data specifying first and second curves obtained by projecting and decomposing a three-dimensional curve onto two adjacent perpendicular intersecting planes in a rectangular coordinate system by approximating the projected curves as straight lines and circular arc segments, finding coordinate values (a.sub.i,b.sub.i) of each i-th (i=1, 2. .. ) partition point P.sub.i from among a plurality of partition points partitioning the first curve into a number of line segments using a ratio of first curve line lengths on each side of the partitioning point, finding respective coordinate values (b.sub.i,c.sub.i) of a point Q.sub.i on the second curve having a coordinate value b.sub.i on a common axis among coordinate axes of said planes, specifying the three-dimensional curve by the three-dimensional coordinate values (a.sub.i, b.sub.i, c.sub.i) (i=1, 2,. .. ), and machining the workpiece in dependence on the three-dimensional curve.