To provide a vacuum pump with excellent suction characteristics and a great exhaust speed.

A pump housing 11 has grooves 11f, 11g defining enlarged suction passages 31, 32 radially outwardly of large lead suction port side screw portions 21c, 22c. The ends of the enlarged suction passages 31, 32 at their exhaust port sides are located near a junction A between large and small lead screw portions. When operating chambers 24, 25 communicated with a suction port 11a with the rotation of male and female screw rotors 21, 22 is increased in volume, these perform suction operation through the enlarged suction passages 31, 32 and when the operating chambers 24, 25 communicated with the suction port 11a is maximized in volume, these ends are partitioned with the engaged portions of both exhaust port side screw portions 21d, 22d.