Recently, the robot assembly industry requires research that deals with flexible parts using human like skills. We discuss cylinder insertion into a hole of a flexible rubber plate. We reported that the magnitude of the insertion force expresses a parabolic field in the three-dimensional space that is composed of position and posture of the cylinder. The parabolic field has a tunneling path from which we can obtain the local minimum insertion force. The tunneling path seems to correspond to the rationality of human skill. This paper proposes a new regression method for searching the tunneling path, using the insertion force of examination points without excessing the minimax force of parabolic center. The method gives us a flexibility for many kinds of size of parts, and the consistency of the insertion task. An experimental results show that the regression method is a possible search method for the tunneling path.