To drastically reduce the temp. difference of the surface of a cooling copper plate by disposing zigzag a fastening bolt boss parts in a cooling water passage and arranging tapered supporting plates.

In the cooling structure composed of a water cooling type mold wall for feeding the cooling water into the cooling water passage 5 formed between the fastened cooling copper plate 1 and back plate 2, the cooling structure is provided with the fastening bolt boss parts 7 positioned at the interval between the cooling copper plate 1 and the back plate 2 and arranged at the cooling copper plate 1 and the supporting plate 3 disposed at the downstream zone side of the fastening bolt boss part 7, and the cooling copper plate 1 and the back plate 2 are fastened with the fastening bolt 4. In this cooling structure, the fastening bolt boss parts 7 are disposed zigzag in the flow passage of the cooling water. Further, the width of the supporting plate 3 is made not more than the diameter of the fastening bolt boss part 7 at the position adjoined to the fastening bolt boss part 7 and formed as tapered state toward the downstream direction of the cooling water. The length 8 of the supporting plate is made equal to not less than 5 times of the diameter of the fastening bolt boss part 7. The each setting pitch of the fastening bolt boss part 7 is made larger than the diameter of the fastening bolt boss part 7.