PURPOSE: To guarantee stress within the limits of yield strength by specifying a relation between the outer diameter of a moving blade and maximum pumping speed.

CONSTITUTION: A rotor 2 is arranged in a casing 1, and plural stationary blades 3 are fixedly secured thereon. A relation between outer diameter D0 of a moving blade and maximum pumping speed Smax is selected to satisfy D0=12.3 Smax0.4W16.5S0.4max. This is calculated by faking into account, conditions such as yield strength of material, for moving blades, exhaust efficiency and passage cross section of gaseous molecules. Accordingly, optimum outer diameter of a moving blade for giving maximum pumping speed with stress of a moving blade held within the limits of yield strength can be obtained.