An interpolation method in an automatic programming, capable of properly determining and programming target movement amounts for individual axes in each interpolation cycle at the time of executing a numerical control program. In the case where the length (P') of the last one of a plurality of sub-sections, obtained by dividing a section from the starting point (A) to the end point (B) of each block in a program by a target movement amount (P) per interpolation cycle, is smaller than the value (P), and when an angle (.theta.) between the paths of the block concerned and the next block is smaller than a reference angle or when the angle (.theta.) is larger than the reference angle and the sub-section length (P') is smaller than a reference length, an intersection at which a sphere having the starting point (an) of the last sub-section as its center and theh value (P) as its radius crosses the target movement path of the next block is derived by the overlapping process, and the interpolation process for the last sub-section is effected with the intersection used as the target end point of the last sub-section. On the other hand, when the angle (.theta.) and the length (P') are larger than the reference angle and reference length, respectively, the interpolation process is effected with the original end point (B) used as the target end point of the last sub-section, without effecting the overlapping process.