PROBLEM TO BE SOLVED: To enhance mechanical strength of on entire casing by enhancing the vacuum holding ability of the bonded part of a transparent window and the casing in a vacuum tube flat-plate type solar collector. SOLUTION: A bottom plate reinforcing plate 10, integrated with a bottom plate 7 of the casing 4, is provided; and a support frame 13 is provided between a peripheral part 5 of the casing 4 and the bottom plate reinforcing plate 10, so as to apply atmospheric pressure on the bonded part of the window 3 and the peripheral part 5.
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