PURPOSE: To effect accurately a correction control by suction air and prevent the deterioration of a humidity sensor by a method wherein a part of exhaust gas is recirculated into a suction system at the upstream of the humidity sensor while a fuel supplying device is provided in the downstream thereof.

CONSTITUTION: The humidity sensor 9 is provided in the downstream of the recirculating exhaust gas C supplied to a suction path 2 to detect the humidity of suction air after the recirculating exhaust gas C is supplied or the suction air actually supplied into a combustion chamber. A fuel injection valve 10, serving as a fuel supplying device, is provided in the suction path 2 at the downstream of the humidity sensor 9. A humidity control device 12 releases moisture D into the suction path 2 at the upstream of the humidity sensor 9.