We discuss cylinder insertion into a hole of a flexible rubber plate, which is an example of research the robot assembly industry requires for dealing with flexible parts using human-like skills. We show that the magnitude of insertion force expresses a hyperbolic field in three-dimensional space composed of position and posture of a cylinder. The hyperbolic field has a tunneling path from which can obtain local minimum insertion force. This path appears to correspond to the position and posture of cylinder insertion using human skill. We propose a search for the tunneling path that uses a simple regression model that fits the hyperbolic field to a narrow zone. The fitting zone shifts as cylinder insertion advances. Results show that regression is a possible search method for the tunneling path.