To correctly and constantly detect an intake air flow rate, even when soot or dust is accumulated on a main flow sensor which is disposed to an intake passage and detection output of the main flow sensor is lowered.

The main flow sensor 21 is disposed to the intake passage 13 of the engine, and a flow rate of intake air passing through the intake passage 13 is detected by the main flow sensor 21. A bypass passage 27 is disposed to the intake passage 13 to bypass the intake passage 13, and an auxiliary flow sensor 31 is disposed to the bypass passage 27 to detect the flow rate of the intake air which passes through the bypass passage 27. Further, a pair of filters 34, 34 are installed to the bypass passage 27 to sandwich the auxiliary flow sensor 31 therebetween, and the detection output of the main flow sensor 21 can be corrected by detection output of the auxiliary flow sensor 31.