Air control device for internal combustion engine

In an air control device including a body, an air passage formed in the body, a throttle valve disposed in the air passage for controlling an air quantity flowing in the air passage, a throttle shaft for mounting the throttle valve and extending across the air passage, and a pair of first and second ball bearings mounted in the body for rotatably supporting the throttle shaft; a supporting structure of the throttle valve including a first shoulder formed in the body; a first stopper fixed to the body; an inner ring of the first ball bearing engaged by interference fit with the throttle shaft; an outer ring of the first ball bearing engaged by clearance fit with the body, the outer ring being fixedly sandwiched between the first shoulder and the first stopper under the condition where a thrust is applied to the outer ring in an axial direction of the throttle shaft; a second shoulder formed in the body; a second stopper engaged by interference fit with the body; an inner ring of the second ball bearing engaged by clearance fit with the throttle shaft; and an outer ring of the second ball bearing engaged by clearance fit with the body, the outer ring being fixedly sandwiched between the second shoulder and the second stopper. Further, the surface of the throttle shaft is treated by a low-temperature salt bath nitriding process, so that fretting wear between the throttle shaft and the inner ring of the bearing engaged by clearance fit with the throttle shaft.