Branch pipes (3) of an intake manifold (1) comprise main bodies (3c), a common flange (11b) and pipe shape joining portions (11c) respectively connecting the main bodies (3c) to the flange (11b). By injection molding, the common flange (11b), the joining portions (11c) and a portion of the main bodies (3c) are formed in one piece as a first component (11), and the remaining portion of the main bodies (3c) is formed in one piece as a second component (12). A notch that has a metal mold draft angle (11d) is formed on the inner circumference of joining portion (11c) and the flange (11b). Filling the notch by using an adjusting member (22) when attaching the intake manifold (1) to an engine through the flange (11b) compensates for changes in shape of a passage cross section caused by the draft angle. A constant passage cross section is thus maintained.