A part of the air flowing through a main passage flows into a by-pass passage. A flow rate detector is provided in the by-pass passage. An outlet of the by-pass passage is opened at the narrowest portion of a venturi structure provided in the main passage. The by-pass passage is in communication with a pressure-accumulating chamber, which is in communication with the main passage at the portion thereof which is on the downstream side of the venturi structure. The pressure of the air flowing backward from the engine is stored in the pressure-accumulating chamber. The pressure in the pressure-accumulating chamber serves as a passage resistance with respect to the flow of the air in the by-pass passage. Accordingly, the flow rate detector permits determining the flow rate of the air, which does not contain a component representative of a backward flow of the air.