To reduce the outward heat conduction in a casting range and to obtain a strand having good quality by providing a laminating layer portion on a side surface composed of a material having lower thermal conductivity in comparison with a material of a mold body, so as to form at least some range.

The cross-sectional area of a hollow part 2 in the mold tube 1 is set larger in the end surface 3 at casting side than the area in the bottom part 4 at the outlet side of the strand. The mold tube 1 is provided with the lamination layer portion 8 composed of the material having lower thermal conductivity in comparison with the material of the mold tube or the base body 5 of the mold tube 1 at the portion A of the mold tube at the height of casting level 7 in the outer side surface 6. The lamination layer portion 8 reduces the heat flow and the outward heat conduction of the mold tube 1 in the height range of the casting level 7 to obtain the higher wall temp. than the temp. at which the solidified shell is formed in the strand. In this way, the surface quality of the steel strand is improved and fine crack near the edge of the mold tube 1 is avoided. The lamination layer portion 8 has almost fixed thickness D1 and wear protecting layer 11 composed of chromium plating having about 80 μm thickness in the inside 10.