PURPOSE: To stabilize continuous casting of a steel strip and to eable change of the strip thickness and the strip width by standing rectangular thin metal sheet on one side surface of flexible metal sheet in the equally closed pitch and connecting the flexible metal sheet in endless state as the inside of a mold.

CONSTITUTION: On one side surface of the flexible metal sheet 4, plural pieces of the rectangular thin metal sheets 1 are stood in the longitudinal direction of the sheet 4 in the equally closed pitch (p) by fitting the one side surface side of the rectangular thin metal sheets 1 in the transverse direction of the sheet 4. The flexible metal sheet 4 is made to be the inside and the rectangular thin metal sheets 11 are made to be the outside and the sheet 4 is connected as the endless state to form the thin metal sheet standing endless belt for forming the mold long side. This endless belts are wound over each one pair of guide rolls and faced, and small gap thin mold is formed between the faced horizontal parts. At the same time, the thin metal sheet standing endless belt for forming the mold short side is formed to constitute the mold short side.