Current user’s requirements for excellent product quality means that producers must discover the isothermal casting conditions which will lead to top operating performance in the continuous Casting process. In response to this requirement, a high-powered A. C plasma arc heating system was adopted for the No. 4 Continuous Caster (No. 4CC) put into operation at Kakogawa Works. This heating system is a single-phase A.C plasma type with two torches. This report describes research on the molten steel heating technique utilized by the A. C plasma system, the effects of tundish atmosphere on the characteristics of plasma electric power, and the counter- measures devised to increase the arc voltage using a suitable torch structure.