Intake manifold for internal combustion engine

An intake manifold for an internal combustion engine, comprises an upstream unit of molded resin, the upstream unit including an elongate collector portion that has an inlet opening for introducing cleaned air, and a manifold portion that includes a plurality of curved tubes that are fixed to one another, each curbed tube having an inlet opening exposed to an interior of the elongate collector portion; a downstream unit of molded resin, the downstream unit including a base portion that is to be connected to a cylinder head of the engine and a plurality of apertured collar portions that are projected from the base portion; and a plurality of sealing members through which leading ends of the curved tubes are hermetically inserted into the apertured collar portions thereby to constitute an entire construction of the intake manifold, wherein the molded resin of the downstream unit has a heat resistance that is higher than that of the upstream unit.