PURPOSE: To absorb thermal deformation of a U-shaped inner tube, by inserting the U-shaped inner tube into an evacuated outer tube made of a light transmitting material in the manner that a bend portion of the inner tube is kept free but its opposite end portions extended in parallel to each other are fixed at one end of the outer tube.

CONSTITUTION: A substantially U-shaped inner tube 3 having a relatively small diameter and including a bend portion 9 is inserted into an evacuated outer tube 2 made of a light transmitting material which has a bottomed end 8 and an opening at the other end thereof that is closed by an end plate 5. A spacing 10 is provided between the bottomed end 8 of the outer tube 2 and the bend portion 9 of the inner tube 3, and opposite ends 3a, 3b of the inner tube 3 are extended through the end plate 5 to the outside of the outer tube 2. The inner tube 3 is provided with heat collecting fins 4, 4'.