To keep a gap formed between the outer diameter side end of a throttle valve 1 and the bore inner diameter face of a throttle body 2 at a desired gap size by using a resin forming die even when the throttle valve 1 and the throttle body 5 are formed of resin in the same resin forming die at the same time.

When the throttle valve 1 and the throttle body 5 are formed of the resin in the same resin forming die at the same time, the throttle valve 1 is formed to be assembled in a bore wall portion 6 of the throttle body 5 at a rotating angle in another position than the fully closed position where it is rotated a preset angle from the fully closed position. The gap formed between the bore inner diameter face of a bore inner tube 31 of the throttle body 5 and the outer diameter side end of the throttle valve 1 can be kept at the desired gap size by using the resin forming die. Thus, the sealing performance of the throttle valve 1 when fully closed is secured and the malfunction of the throttle valve 1 is prevented.