The strength development of the steelmaking slag mixed marine clay was investigated when the water content of the clay was lower than the liquid limit, wL. In addition, a method to obtain the same strength as the strength of a specimen with a diameter of 100 mm and a height of 200 mm, which is the standard for steelmaking slag mixed soil, with a specimen with a diameter of 50 mm and a height of 100 mm. In the sample mixed with a hand mixer, the strength of the sample with an initial water content of 0.9 wL was lower than that of the sample with 1.1 wL. However, in the sample mixed with a forced twin-screw mixer, the strength increment accompanied by the decrease in the water content was observed even if the initial water content was 0.9 wL. It is considered that the difference in mixing quality caused the difference in strength. The strengths of the specimens with a diameter of 50 mm and a height of 100 mm prepared by the two proposed methods were 1.12-1.13 times the strength of the standard sample, showing a good correlation. These methods are considered to be effective from the viewpoint of improving the efficiency of research of this material.