This invention relates to an interpolation method in automatic programming which determines appropriately a target moving distance of each axis in each interpolation period at the time of execution of a numerical control program, programs it and can improve a machining speed and machining accuracy. When the length (P') of the last sub-zone among a plurality of sub-zones obtained by dividing a zone from a start point (A) to an end point (B) in each block of a program by a target moving distance (P) per interpolation period is smaller than the value (P); and when an angle (Υ) between target moving orbits in the block described above and the next block is smaller than a reference angle or when the angle (Υ) described above is greater than the reference angle and the length (P') of the sub-zone is smaller than a reference length, the point of intersection between a sphere having the center at the start point (an) of the last sub-zone and a radius of the value (P) described above and the target orbit in the next block is determined by overlap processing and interpolation processing for said zone is made with this point of intersection as the target end point of the last sub-zone. When the angle (Υ) and the length (P') are greater than the reference angle and the reference length, respectively, interpolation processing is conducted by use of the first end point (B) as the target end point of the last subzone without using the overlap processing.