To set at an optimum value a clearance (full-close clearance) formed between the bore inner peripheral surface 13 of a cylindrical portion 2 of a throttle body 1 and the outer peripheral side end face 23 of a throttle valve 4 even if the throttle body 1 and the throttle valve 4 are made of resin.

At a full-close position of the throttle valve 4, a thermoplastic resin product (work) in such a state that the outer peripheral side end face of the throttle valve 4 is connected to the bore inner peripheral surface 13 of the cylindrical portion 2 of the throttle body 1 is cut by a laser to thereby separate the throttle valve 4 from the cylindrical portion 2 of the throttle body 1 so that the throttle valve 4 has a predetermined circle shape. Thus, since the clearance (full-close clearance) formed between the cylindrical portion 2 of the throttle body 1 and the outer peripheral side end face 23 of the throttle valve 4 can be set at the optimum value, the throttle valve 4 is prevented from biting the bore inner peripheral surface 13 of the cylindrical portion 2 of the throttle body 1.