PURPOSE: To uniformly deprive heat from a solidified shell by arranging a partition wall plate for dividing a cooling chamber into two in the radius direction and forming multiple number of through-holes, from which plugs can be attached/detached to/from the wall part in the partition wall plate.

CONSTITUTION: The partition wall 7 is arranged in the cooling chamber 8 outside of the inner wall 2 of a mold and the cooling chamber 8 is vertically divided into two by a flange 6. In the partition wall 7, the multiple number of the screw holes 10 penetrated so as to match to the plug 9 are arranged. Cooling water flows into the lower part of the cooling chamber 8 from an introducing hole 17 and then into a cooling water flowing passage 15 from the screw holes 10 in the partition wall 7 and the lower end of the partition wall 7, and ascends while cooling the inner wall 2 of the mold and come to a drainage hole 18. By screwing the plugs 9 into the screw holes 10 in the partition wall 7, the flow rate in the cooling water flowing passage 15 can partially be controlled and made to cope with the change of conditions, such as kind of steel, casting speed.