The cross-sectional area of a reduced passage increases at a constant ratio from a maximum reduction part where a cross-sectional area of the reduced passage becomes the smallest toward a downstream side. The cross-sectional area of the reduced passage is equal to the cross-sectional area of a bypass passage at a downstream end of a cross-sectional area increasing region intersecting with a wall surface of the bypass passage. (Cross-sectional area increase ratio)=(B−A)/X is satisfied. A [mm2] is the cross-sectional area of the reduced passage at an upstream end of the increasing region. B [mm2] is the cross-sectional area of the reduced passage at the downstream end of the increasing region. X [mm] is a distance from the upstream end to the downstream end of the increasing region along an air flow direction through the reduced passage. The ratio is 1.5 [mm2/mm] or smaller.