To easily perform calking work and changing work of the lower rail.

A lower rail 30 consists of the first rail member 38, which is installed as attachable and removable to an insulated box body 12 with a screw 36, and the second rail member 40, which is installed as attachable and removable to the first rail member 38. The first rail member 38 is equipped with a support part 42, which is positioned as corresponding to the insulated box body 12, and a engaging part 46, which is combined with the second rail member 40. Two rail streaks 58 and 60 are set up on the upper face of the horizontal connecting board 56, which connects a front board 52 and a rear board 54 of the second rail member 40. By combining an engaged part 64, which is formed in the lower part of the front board 52 and the rear board 54 of the second rail member 40, with the combining part 46 of the first rail material 38, the second rail member 40 is installed to the first rail member 38.