In a rotor blade cylindrical portion or a stator portion in which thread grooves are formed, convex surfaces (thread groove peak surfaces) configuring the peak sections of the thread grooves are cut by a desired amount over the entire circumference (a circumferential direction of the thread grooves) of a certain range in an axial direction. As a result, the gap on the lower side of the thread groove portion can partially be expanded by cutting the thread groove peak surfaces by a desired amount.