PURPOSE: To prevent entering of carbon and oil in a by-pass passage, by providing a plate material bearing dynamic pressure of an air flow at an upstream side of an inlet of a pipe disposed at the by-pass passage.

CONSTITUTION: A plate material 10 bearing dynamic pressure of an air flow is fixed at an upstream of an inlet of a pipe 9 and an inlet of the pipe 9 is opened at a downstream side almost in the center of the plate material 10. Therefore, the air having small inertia reaches the downstream side by making a round at the side surface from the upstream side of the plate material 10 and enters from the inlet opening part of the pipe 9. While, in case of dust, carbon and an oil drop having large inertia, the direction of them is changed by the plate material 10 and they hardly enter the inlet of the pipe 9 opened at the downstream side of the plate material 10 and carbon, etc. do not enter a by-pass passage 6.