PURPOSE: To suppress cavitation as quickly as possible by applying rubber lining processing on a core bar member of 3-vane Roots rotors in a Roots pump and providing small-diameter communicating ports for preventing cavitation in respective arc parts of the rotors.

CONSTITUTION: In a Roots pump for sewage which is formed by storing a pair of three-vane Roots rotors 10 in a casing provided with a suction port and a delivery port, core bar members 12 which have a clearance part formed in an appropriate position by imitating the rotor's shape are provided on respective rotors. And rubber lining processing (L) is applied on the clearance parts and the surface of the core bar members. Small diameter communication ports 17 for preventing cavitation and closing phenomena are radially bored in recessed arc part 16 between respective projecting arc parts 15 of the Roots rotors which are arranged at 120 intervals. And a ring cutter board 20 whose outer circumference is formed into a continuous recessed/projecting cutter part 21 is fixed in the central position of respective sides of respective rotors 10 by a bolt.