Legged robots have been studied and developed as a mobile robotic platform.In many types of legged robots,a quadruped robot has both compactness and stability.To make a quadruped robot be useful,it is required to walk dynamically and to move fast.We propose an algorithm to achieve a dynamic gait pattern.The dynamic gait which we deal with is trot gait that the robot swings the pair of the legs on the diagonal line at the same time.Generally,in order to estimate the motion of the robot walking dynamically,we have to calculate the complex dynamics.Therefore we model the quadruped robot by an inverted pendulum in order to simplify the equation of motion of the robot.Then we generate the trot gait patterns by using the equation of motion of the inverted pendulum.The quadruped robot does not always walk correctly.Therefore we propose the swing leg trajectory which is composed of ellipse and straight lines in order to reduce the influence of wrong behaviors of the robot.We conducted experiments that the quadruped robot was walked by the trot pattern which we made.As a result,the quadruped robot walked successfully.