In a continuous casting apparatus 100 for casting a stainless steel billet 3c, a long nozzle 2 extending into a tundish 101 is provided at a ladle 1 for pouring a molten stainless steel 3 in the ladle 1 into the tundish 101. Further, a nitrogen gas 4 is supplied as a seal gas around the molten stainless steel 3 in the tundish 101, and continuous casting of the stainless steel billet 3c is performed, in which, while immersing the spout 2a of the long nozzle 2 into the molten stainless steel 3 in the tundish 101, the molten stainless steel 3 is poured through the long nozzle 2 into the tundish 101 and the molten stainless steel 3 in the tundish 101 is poured into a casting mold 105.