To ensure sealing property relative to the outside air and to relax electric potential concentration at metal flanges, by providing sealing members airtightly jointed to an FRP tube on the inner bore side at both ends thereof and electric field relaxing parts projecting in the minor diameter side of the FRP tube.

Sealing parts 11 are jointed to an FRP tube 2 on its inner bore side at both end parts of the cylindrical FRP tube 2 and curvature parts 12 protruding from the sealing parts 11 respectively toward the interior of the FRP tube 2 are provided and jointed to the sealing members 11. Because the sealing members 11 are each jointed airtightly to an inner circumferential part of the FRP tube 2 with an adhesive or the like, sealing property between the FRP tube 2 and the sealing members 11 is ensured. Further, construction is made such that an end part 12a of each of the curvature parts 12 is positioned to exceed an end part 6a of each of the metal flanges 6 on the outer circumferential surface of the FRP tube 2, and therefore an electric potential gradient is reduced in the vicinity of the end parts 6a of the metal flanges 6 and electric potential can be prevented from concentrating at these parts.