In this paper, a real-time walking trajectory generation method at constant body height in a single support phase for three-dimensional biped robots is proposed. An analytic solution of a body trajectory was obtained from a ZMP (Zero-Moment Point) equation of a body and a swing leg trajectory. Real-time body trajectory is generated based on an analytic solution. This body trajectory is applied, the biped robot walks stably without up and down of the body height. Additionally, the modeling of the proposed method is more precisely than that of the conventional methods. The validity of the proposed method was confirmed by some simulations and an experiment.