To properly adjust an air-fuel ratio by fixing an air-fuel ratio adjusting valve firmly and stably to a prescribed adjusting position.

In a fitting hole 11 formed by dividing screw holes 8 front and rear, an elastic material-made fixing member 21 is press fitted without a space, passing a screw through hole 22 having an internal thread 23 formed by rolling of the fixing member 21, a screw bar 17 of an adjusting valve 15 is screwed in the screw hole 8. The internal thread 23, by elastic restoring force of itself, is brought into total surface press contact with an external thread 20 of the screw bar 17, so as to prevent generation of a chatter and fuel leakage.