A resonator made of a synthetic resin provided in an intake path of an engine, comprising: an air chamber formed by melt-welding a circumferential wall portion of a first box forming body and a circumferential wall portion of a second box forming body, while abutting the circumferential wall portions (4b, 5b) against each other, the first box forming body including a bottom surface portion and the circumferential wall portion surrounding the bottom surface portion, and the second box forming body including a bottom surface portion and the circumferential wall portion surrounding the bottom surface portion, wherein a circumferential groove is formed on an upper end portion of the circumferential wall portion of the first box forming body, and a circumferential protrusion is formed in an upper end portion of the circumferential wall portion of the second box forming body, the circumferential protrusion being melt-welded to a groove bottom of the circumferential groove by being received in the circumferential groove during abutment, and an upper end surface between an outer wall surface in the circumferential wall portion of the first box forming body and the circumferential groove is located above the upper end surface between an inner wall surface and the circumferential groove, characterized in that the circumferential groove of the first box forming body has a deepest portion on one groove wall located inside the first box forming body, and has an inclined bottom surface that makes the circumferential groove shallow as it goes toward the other groove wall between the deepest portion and the other groove wall, and the circumferential protrusion is adapted to be melt-welded by being pressed against the inclined bottom surface so that the welding can be performed in such a manner that the molten resin caused by the welding is induced to the inside of the first box forming body by the inclined bottom surface, to prevent a leakage of the molten resin to the outside of the resonator, wherein the circumferential groove forms a groove bottom in the deepest portion as a horizontal plane in a direction perpendicular to the inner wall surface, and forms the groove bottom between the deepest portion and the other groove wall as the inclined bottom surface. an air chamber formed by melt-welding a circumferential wall portion of a first box forming body and a circumferential wall portion of a second box forming body, while abutting the circumferential wall portions (4b, 5b) against each other, the first box forming body including a bottom surface portion and the circumferential wall portion surrounding the bottom surface portion, and the second box forming body including a bottom surface portion and the circumferential wall portion surrounding the bottom surface portion, wherein a circumferential groove is formed on an upper end portion of the circumferential wall portion of the first box forming body, and a circumferential protrusion is formed in an upper end portion of the circumferential wall portion of the second box forming body, the circumferential protrusion being melt-welded to a groove bottom of the circumferential groove by being received in the circumferential groove during abutment, and an upper end surface between an outer wall surface in the circumferential wall portion of the first box forming body and the circumferential groove is located above the upper end surface between an inner wall surface and the circumferential groove, characterized in that the circumferential groove of the first box forming body has a deepest portion on one groove wall located inside the first box forming body, and has an inclined bottom surface that makes the circumferential groove shallow as it goes toward the other groove wall between the deepest portion and the other groove wall, and the circumferential protrusion is adapted to be melt-welded by being pressed against the inclined bottom surface so that the welding can be performed in such a manner that the molten resin caused by the welding is induced to the inside of the first box forming body by the inclined bottom surface, to prevent a leakage of the molten resin to the outside of the resonator, wherein the circumferential groove forms a groove bottom in the deepest portion as a horizontal plane in a direction perpendicular to the inner wall surface, and forms the groove bottom between the deepest portion and the other groove wall as the inclined bottom surface.