PURPOSE: To improve the delay in the temp. follow-up of the pipe wall of a conduit for measuring the amt. of suction air and permit stable detection of the amt. of air despite abrupt change in the temp. of the suction air by providing a partition wall segmenting upper and down streams of suction air on the down stream side than a suction air detecting part.

CONSTITUTION: The suction air introduced through an air intake port 5 passes through a cleaner element 6, and contacts with the outside surface of a conduit 2 for measuring the amt. of suction air, after which it is rectified with a rectifier 7 and is introduced into the conduit 2. Since the inside and outside surfaces of the pipe wall of the conduit 2 contact with suction air in this way, the follow-up characteristic of the pipe wall temp. is extremely improved, and in spite of the transfer of the temp. of the pipe wall in the down stream part of the conduit 2 to an air cleaner case 1 through a partition wall 9 provided on the down stream side than a suction air detecting part 4, no adverse influence is given to the detection of the amt. of suction air.