To eliminate the effect of the centrifugal force on a valve element of a check valve.

A valve seat 30a of a check valve 29 is formed of a tapered surface. A generator farthest from the axis L of rotation among the generators of the tapered surface to form the valve seat 30a is orthogonal to a radius R of a rotor. When a ball valve 31 is seated on the valve seat 30a, the ball valve 31 is brought into contact with the interference of the generator with the radius R.