To prevent oil leaks between a valve plate and a cylinder block more reliably without causing the support ends of a center shaft and the support ends of piston rods to separate from the end face of a drive shaft.

A center shaft 90 is equipped with: an outer race 920, which is mounted in a shaft-mounting hole 41 of a cylinder block 40, and has a shaft-part-housing hole 921 on one end and a spring-housing hole 922 on the other end; and an inner shaft 910, which has at the tip of a shaft base part 911 a shaft support ball head part 912 having a large outer size, and which is mounted in the shaft-part-housing hole 921 of the outer race 920 by means of the shaft base part 911. With a compression spring 930 housed in the spring-housing hole 922, one end of the outer race 920 is mounted in the shaft-mounting hole 41 of the cylinder block 40, and the center shaft is supported in a tiltable manner on the end face of the drive shaft 30 by a retainer plate 100, via the shaft support ball head part 912 of the inner shaft 910.