To sufficiently correct a curve even when autoclave sterilization is performed.

A piston 28 is driven by a pressure difference between the inside of a chamber and the inside of a cylinder 29, and a slider 27 is slid in a lengthwise direction by a rod 28a connected to the piston 28. An inserting part 16 of an endoscope held by the slider 27 is slid in the lengthwise direction along with an inserting part housing groove 25, and the curved form of the inserting part 16 is forcibly corrected. While the endoscope 15 is cooled from 120°C or more to about an ordinary temperature, the piston action is repeated several times and the endoscope is cooled while continuously changing the curved form of the inserting part 16, so that the fixed curve can not be formed.