A quadruped robot that cannot only walk but also move by rolling its body is proposed. The robot has a spherical body. Two legs are attached to the ends of the body’s central axis. On irregular terrain, the robot walks in the same manner as general quadruped robots. On flat terrain, the robot places its body on the ground and rolls it around its central axis using its legs. When the robot travels straight, the central axis is parallel to the ground, and the legs on both sides move in the same manner. Initially, all four legs are grounded. First, the robot swings up both front legs. Subsequently the robot’s center of gravity shifts forward, causing the body to naturally roll forward. As a result, the front legs will land forward. The robot swings down two hind legs and lands. Second, the robot lifts the hind legs and moves them forward. The body rolls forward by pushing the ground with its front legs. Third, the robot lifts its front legs. Subsequently the body naturally rolls again, and the hind legs land forward. Finally, the robot lands on its front legs. In this manner, the robot’s front and hind legs are switched, and it returns to its original state. Repeating the process allows for continuous rolling. The robot can turn by tilting the central axis of the body. This is achieved when the heights of the right and left legs differ. A prototype robot is developed, and its rolling speed and turning radius are evaluated.

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