PURPOSE: To regulate movement of rotors in the thrust direction by facilitating manufacture with two pump units integratedly clamped in the axial line direction by a clamping means and interposing an elastic member between axial ends of the rotors pressed to make contact with each other by an energizing means.

CONSTITUTION: Two pump units 2, 3 are clamped in their axial line direction by three bolts 35 to constitute an integrated two-step pump 1. Very small clearances are formed between right and left sides of rotors 4-2, 4-3 and vanes 10-2, 10-3 and end plate blocks 6-2, 6-3, 7-2, 7-3, but the pump units 2, 3 are constructed in two-step, then air leak in the clearances can effectively be restrained to obtain high degree of vacuum. And movement of the rotors 4-2, 4-3 in the thrust direction is regulated by a gland 27 and spring washers 26, 28, and mutual contact between side faces of the rotors 4-2, 4-3 and end plate blocks 6-2, 6-3, 7-2, 7-3 can be prevented.