PURPOSE: To reduce incurring of a power loss during transfer from non- supercharging operation to supercharging operation, to improve moldability and assembling ability, to form a device in a compact manner, and to reduce a cost.

CONSTITUTION: A communication hole 7 communicated with an upper stream intake air passage is formed in the side wall 2b of a housing 2, the side wall corresponding to the starting position of engagement between a screw groove 3a and a screw protrusion 4a of female and male rotors 3 and 4, respectively. A delivery port 8 is communicated with an upper stream intake air passage through a gap, between the screw groove 3a and the screw protrusion 4a, engagement between which is progressed through rotation of the female and male rotors 3 and 4, and a communication hole 7. A valve means which is closed during supercharging operation of an engine to close the communication hole 7 and opened during non-supercharging operation of the engine to release the communication hole 7 is located in the communication hole 7.