PURPOSE: To sufficiently reduce the generation of peak noise owing to the occurrence of a carrier overflow.

CONSTITUTION: A passage inlet 13b is formed in a partition wall 4a in a position situated at a distance of approximate 0.5 t from a partition wall end 4b. A part of a carrier overflow flows through the passage inlet 13b to a passage 13 and flows out to a suction port 3 after the passage of it through a passage 13. The carrier overflow is interfered with a remaining carrier overflow flowing through the partition wall end 4b to the suction port 3 along a conventional route. A flow rate of a bypass flow is set by means of an orifice 14 such that the amplitude of a pressure waveform generated from a passage outlet 13a is caused to coincide with the amplitude A of a pressure waveform generated from the partition wall end 4b. This constitution causes realization of ideal interference wherein the same waveforms are overlapped with each other in a state to be displaced from each other by a halfwavelength, and damping of peak noise.