PURPOSE: To prevent the generation of closing phenomenon of sealing water by forming a transverse hole parallel to a shaft at the head part of a Roots rotor and installing a pressure-sensitive deformation member into said transverse hole and forming the sealing-water ports which communicate to the transverse hole onto the peripheral surface of the head part.

CONSTITUTION: A transverse hole 7 parallel to a shaft is formed at the head part 5 of a roots rotor 4, and a pressure-sensitive deformation member 12 consisting of a pipe or a solid rod is inserted into the transverse hole 7. Sealing water ports 9W11 which communicate to the transverse hole 7 are formed onto the peripheral surface of the head part 5. Therefore, the sealing water supplied into a casing 1 can intrude inside from the sealing water ports 9W11 formed onto the peripheral surface of the head part 5 of the Roots rotor 4 by the deformation of the pressure-sensitive deformation member 12, and the sealing water having a high pressure is safely released into the transverse hole 7 in the head part 5 of the Roots rotor 4.