The Spring Loaded Inverted Pendulum (SLIP) describes the dynamic walking of humans and animals in a simplified manner. However, realizing such movements by means of a combination of typical articulated legs and linear actuators has some limitations. This paper proposes a leg mechanism that accurately reflects the SLIP based on its mechanical constitution. The SLIP design based leg is able to do decoupled swing motion and extension motion. Also this study has focused on improving the straightness of extension motion.