PURPOSE: To prevent suction of the air into molten steel flowing out through a nozzle and to prevent the development and the growth of blowhole in the steel with inert gas.

CONSTITUTION: The sliding nozzle 3 is communicated with a tundish 1. This sliding nozzle 3 is connected with a mold 5. In outer periphery of the above sliding nozzle 3, a sealing chamber 6 having the prescribed space and covered with iron plate, is formed. Cylinders 7a, 7b for argon gas and hydrogen gas are connected with this sealing chamber 6. At the time of pouring the molten steel 2 into the mold 5 from the above tundish 1, before starting the pouring, the argon gas is beforehand flowed into the sealing chamber 6. Then, after pouring the molten steel 2, the hydrogen gas instead of the argon gas is flowed into the above sealing chamber 6 in the same quantity, and after that, the hydrogen gas is further flowed so that inner pressure in the sealing chamber becomes 1.2kg/cm2.