Human is capable of adaptive and supple locomotion in the real world characterized by rapid changes, high uncertainly, and limited availability of information. In order to understand the human walking, this work was motivated by the concept of passive dynamic walking robots, which have no actuation or control system except for gravity are capable of stable, human-looking walking. On the other hand, human can produce a stepping motion not only depend on the legs, but also the rotation of the Center of Mass, arm-swing, the motion of the torso and so on. In this paper, a three dimensional quasi-passive dynamic walking provoked by rocking motion in lateral plane has been investigated. The behavioral analyses with the robot experiments show that this robot can walk on a flat ground and a gait speed is related to the period of lateral rocking.