Specifically, the heat-discharge structure 43 is a structure of conducting heat from an inner wall part 41A to the corresponding lower wall part 42A to discharge the heat outward. The heat-discharge structure 43 typically causes condensing heat occurring at an inner wall part 41A to transfer to the corresponding lower wall part 42A, so that the heat is discharged outward.

The heat-discharge structure 43 according to the present embodiment includes a heat-transfer sheet 44. The heat-transfer sheet 44 is tabular (sheet-shaped) and is interposed between an inner wall part 41A and a lower wall part 42A in contact with the inner wall part 41A and the lower wall part 42A, which conducts heat from the inner wall part 41A to the lower wall part 42A. Provided are heat-transfer sheets 44 each interposed between the corresponding inner wall part 41A and the corresponding lower wall part 42A. That is, in the example of FIG. 2, provided are two heat-transfer sheets 44 in total, one of the two heat-transfer sheets 44 being interposed between one of the pair of inner wall parts 41A and the corresponding lower wall part 42A of the pair of lower wall parts 42A, the other heat-transfer sheet 44 being interposed between the other inner wall part 41A and the other lower wall part 42A.