As a central pattern generator, a neuro-oscillator with four neurons was used for the motion generation of a biped locomotive robot. There is a problem to determine unknown parameters in nonlinear differential equations of the neuro-oscillator so that it generates joint trajectories which yields reasonable biped locomotion. For this problem, we apply the describing function method for the neuro-oscillators, then we obtain equations which should be satisfied for the existence of a periodic solution. Adding a walking pattern which is represented by a trajectories of swinging leg, we can determine the unknown parameters from the equations obtained above. The effectiveness of the proposed method as a design procedure of the biped locomotion was assured by simulation study and experiments with a small biped robot.