To prevent longitudinal cracking easily occurring at an initial stage of casting when performing continuous casting of a medium carbon steel.

When continuously casting the medium carbon steel having a C content of 0.08-0.18 mass%, the basicity of body mold powder injected in a casting mold in a steady state is set to 1.2 or more. The basicity of initial mold powder injected in the casting mold at the initial stage of casting is made larger than that of the body mold powder. The basicity of the initial mold powder is set to 1.9 or less. The injection amount of the initial mold powder is set to satisfy: an injection amount (kg)≥3.0×(inner dimension [m] of upper part of casting mold+inner thickness [m] of upper part of casting mold).