A supercharger (15) is provided with a bypass pathway (17). A backflow preventer (20) is provided to the air intake pathway (8) between an airflow meter (10) and the bypass pathway (17). The backflow preventer (20) has: a casing (41) formed by locally expanding the cross-sectional area of the air intake pathway; and an inner tube (42) substantially constructing a duct for the mainstream of air intake. The inner tube (42) is supported by a flange portion (50) and the downstream end (42a) opens within the casing (41). During backflow when the bypass pathway (17) opens, a portion of the intake air enters into an exit-side space section (55), and the flow is prevented by the flange portion (50). Consequently, reverse flow is suppressed.