Throttle body with internally connected double tube structure

A resin throttle body (5) has a bore wall part (6) having a double tube structure formed by an inner bore tube (31) and an outer bore tube (32) connected to each other via an annular connection part (33). The annular connection part (33) has an axial wall thickness which is set to be smaller than the minimum radial thickness of a portion of the inner bore tube (31) and a portion of the outer bore tube (32) disposed around the annular connection part (33) are. Therefore, the annular connection part (33) can be formed with a thin-walled radially elongated plate shape. The annular connecting part (33) has a radial cross-sectional length which is greater than its axial wall thickness, so that the rigidity and the strength of the annular connecting part (33) are reduced. Therefore, contraction of the outer bore pipe (32) that occurs in its casting process does not severely affect the inner bore pipe (31), so that deformation of a cylindrically shaped inner periphery (8) of the inner bore wall (31) that rotatably receives a disk-shaped throttle valve may be limited ,