A resinous throttle body has a bore wall part having a double pipe structure constructed with a bore inner pipe and a bore outer pipe that are connected with each other via an annular connecting part. The annular connecting part has axial board thickness set to be less than the minimum radial thickness of a portion of the bore inner pipe and a portion of the bore outer pipe that are located around the annular connecting part. Therefore, the annular connecting part can be formed to be in a thin-walled radially elongated plate shape. The annular connecting part has a radially cross-sectional length larger than its axial board thickness, so that rigidity and strength of the annular connecting part is decreased. Therefore, contraction of the bore outer pipe occurred in its molding process does not largely affect the bore inner pipe, so that a cylindrical-shaped inner periphery of the bore inner wall, which rotatably receives a disc-shaped throttle valve, can be restricted from being deformed.