PURPOSE: To make it possible to form a hole for a neck part at correct position and in correct hole diameter by a method wherein a resonator is formed by integrally welding a tank upper side and a tank lower side together through a connecting flange part and attaching wall extension parts are formed at both the axial ends of a hose attaching wall under the condition that the extended ends of the attaching wall extension parts are formed outside the projected ends of the connecting flange part.

CONSTITUTION: A resonator 1 has weldable connecting flange parts 3 and 13 and consists of a tank upper side 2 equipped with a hose attaching wall 4 and a tank lower side 12, both of which are molded separately. On the tank upper side 2 side, to which a hose main body 21 is attached, a hose attaching wall 4 is formed. On the attaching wall 4, attaching wall extension parts 5, the width b2 of which is formed longer than the width b1 of the connecting flange part 3, are formed so as to allow to release the connecting flange part 3 from a blow mold with no interference between the flange part and the mold after the blow molding of the hose main body 21. Under the condition that the tank upper side 2 is set on a blow mold 31, the hose main body 21 is blown so as to be integrally connected with the tank upper side 2 in order to form the hole for a neck part on the attaching wall. By welding the connecting flange parts 3 and 13 together, the tank upper side 2 and the tank lower side 12 are made into an integral body.