This paper introduces a new motion planning algorithm for a biped robot. A geometric constraint equation derived by reshaping the existing ZMP equation enables one to treat the kinematics of the biped robot as a "redundant degrees of freedom" system. Using the ZMP constraint equation, a sequential redundancy resolution algorithm is proposed, which ensures the ZMP trajectory, the planned operational motion, and additional sub-criteria such as joint limit index. The feasibility of the proposed algorithm is verified by simulating a human scaled 12-DOF biped robot.