The compensational motion by the upper part of the body for the biped walking robot at transition when the walking velocity varies is investigated. The proposed method in this paper calculates the compensational motion of the upper part of the body at the transition from the motions of that at the stationary states before and after the transition. The motions at the stationary states are derived from the calculation using ZMP (Zero Moment Point). If the number of steps at the transition is a few, the proposed method enables the robot to walk stably at the transition even when the walking velocity varies much. In addition the sole's size necessary for the stable walking is derived from the area of ZMP.