To provide an opening area setting device capable of effectively preventing the alteration of a screwing amount of an adjusting member.

An opening area setting device 1 has a hole 8 of which one end is connected to a feeding channel 5 feeding mixed gas and of which the other hand is opened and which has a female screw part 11 and a male screw part 12 threading with the female screw part 11, and is equipped with an adjusting member 9 protruding into the feeding channel 5 and deciding an effective opening area of the feeding channel 5, and an adhesive 10 covering the rear end fringe of the member 9. The rear end side end fringe of the adjusting member 9 is located in the connecting end side more than the opening end of the hole 8, an inner wall groove 15 extending from between both of the end parts of the female screw part 11 to the opening end between both of the end parts of the male screw part 12 is arranged in the inner wall of the hole 8, and the adhesive 10 permeates the inner wall groove.