Line changeover device for endoscope

A line changeover device for an endoscope, which changes over a line which is arranged in the endoscope, is arranged such that it comprises a cylinder and a piston which is fitted and inserted into the cylinder in a retractable manner, a seal member which is abutted against the cylinder for retaining at least one of water tightness and gas tightness with respect to the cylinder is fixedly provided on an outer periphery of the piston, and a strength reinforcement is provided on at least a part between an outer periphery of said seal member and an inner periphery thereof. The water tightness and the gas tightness within the cylinder are retained or held, and a sliding resistance of the piston is kept appropriately or adequately without the use of lubricating oil. Sliding ability between the cylinder and the piston at the time of operation is improved. Operability can be made fine or satisfactory.