This paper presents a design method of the forming groove-to-separating groove rolling sequence used in slit rolling process. Based on the equivalent rectangle approximation, we have designed the forming groove which produces a peanut-shaped cross section via rolling from workpiece with an inlet (incoming) square cross section. The separating groove is designed by lowering its width and height in the ratio of 1:3 for the given reduction ratio. With the forming groove and separating groove designed by the proposed design method, we have performed a pilot rolling test at high temperature (around 1000$^{circ}C$) using low carbon (0.1%C) steel and have also carried out finite element analysis to examine the expansibility of the proposed design method. The Results reveal that if the ratio of the roll diameter over the side length of a square is in the range of 4.0 ~ 8.0, the design method proposed in this study can be systematically applied to design the forming groove-to-separating groove without relying on in-house databases and/or on-site experience.