To make intake air smoothly flow inside a synthetic resin-made intake manifold by smoothly absorbing a dislocation to be generated between an abutment surface inside edge of a downstream side part and an abutment surface inside edge of an upstream side part when welding.

Vibration welding of a synthetic resin path structuring member 15 to an engine side part 18 having a nearly semi-circular cross section is divided into an upstream side welding part and a downstream side welding part. In the upstream side welding part, the abutment surface inside edge 15a1 of the upstream side part 15a of the synthetic resin path structuring member is formed to have a diameter larger than that of the abutment surface inside edge 19a1 of the downstream side part 19a of the engine side part having a nearly semi-circular cross section, and the abutment surface inside edge of the upstream side part is positioned outside the intake flow path more than the abutment surface inside edge of the downstream side part. In the downstream side welding part, the abutment surface inside edge 19b1 of the upstream side part 19b of the engine side part having a nearly semi-circular cross section is formed to have a diameter larger than that of the abutment surface inside edge 15b1 of the downstream side part 15b of the synthetic resin path structural member, and the abutment surface inside edge of the down stream side part is positioned outside the intake flow path more than the abutment surface inside edge of the upstream side part.