In this presentation we propose an algorithm for the adaptive gait control by which a quadruped walking robot can walk against external disturbances. The proposed gait mechanism makes it possible for a quadruped walking robot to change its gait pattern in response to external disturbances from various external forces acting on the body of the walking robot. More specifically, we propose a new criterion for the stability margin of a walking robot under unknown external forces by using the effective mass center based on the zero moment point. And for the solution of the adaptive gait algorithm against external disturbances, a method of altitude control and reflexive direction control is suggested. Based on the algorithmic search method, an optimal stride of the quadruped walking robot that walks from any leg configuration to the standard configuration is also proposed. To verify the efficiency of the proposed approach, some simulation results are provided.