The invention relates to an intake air flow sensor for internal combustion engines comprising a sensor main body (12) and a connection unit main body (14), the former consisting of a main passage (15) for passing intake air and a bypass passage (16) for bypassing part of the air flowing through the main passage (15) and joining it again to the air of the main passage (15). An intake air flow meter (26) is disposed in the bypass passage (16) to measure the intake air quantity. The bypass passage (16) has an outlet opening (20) which opens to the main passage (15). A ring (22) is fitted into the main passage (15) of the sensor main body (12) in such a manner as to close part of the outlet opening (20) of the bypass passage (16). An insulator (30) is clamped by the ring (22) and the connection unit main body (14). According to this construction, the area of the opening (20) of the bypass passage (16) to the main passage (15) is not changed due to the deformation of the insulator interposed between the sensor main body (12) and the connection main body (14) which are fixed together by bolts.