In recent years, studies of biped locomotion have been developed rapidly. Biped robots are able to walk smoothly. In addition, there came some robots which were able to run. Walking and running will be basic performances of biped robots in future. In this paper, we propose a new trajectory planning method, applicable to plan both walking and running motions. The two kinds of motions are treated as a unified trajectory. The trajectory consists of two phases; sinusoidal trajectory and parabolic trajectory. With this method, we plan walking and running motions for a biped robot in a unified manner, and switch between walking state and running state by parameter modification. We simulated and experimented with the proposed trajectory planning method.