PURPOSE: To prevent the adhesion and formation of deposits of an undesirable polymer by forming an internal liner as a sleeve which is formed concentric with a cylindrical outer wall and is formed of one member sheet and making the radial thickness of the internal linear smaller than the wall thickness of the cylindrical outer wall.

CONSTITUTION: This structure is provided with the cylindrical outer wall 11 of the one member and the cylindrical inner liner 20 which consists of the planar sheet of the one member, has a smoothly continuous inside surface and exists apart radially on the inner side from the outer wall 11. A flow passage 24 for a coolant between the outer wall 11 an the inner 20 is formed by a supporting means 21 installed in the space between the outer wall 11 and the liner 20. The supporting means 21 is fixed to the outer surface of the liner 20 by a welding metal 22. Further, the liner 20 is formed as the sleeve which is formed concentric with the outer wall 11 and is formed of the one member sheet. The thickness in the radial direction of the liner 20 is set smaller than the wall thickness of the outer wall 11. Consequently, the smooth inside surface for preventing the adhesion and formation of the deposits of the undesirable polymer may be obtd.