PROBLEM TO BE SOLVED: To improve durability and life by inhibiting air tightness failure caused by inclination of a piston while securing smooth operation.SOLUTION: A plurality of packings 74a-74d are fitted to the side surface of a shaft body part 51b of a piston 51 for switching the communication and interception of a plurality of pipe lines. Out of these packings 74a-74d, the end packings 74a, 74d disposed at the axial-direction end of the shaft body part 51b have interferences which are set larger than those of intermediate packings 74b, 74c disposed at positions other than the axial-direction end. Thus, smooth operation can be performed, and air tightness failure caused by inclination of the piston 51 can be inhibited, so that durability and life can be improved.
