In this paper, we consider a stand-up motion from a face-up lying posture by means of swing-down movement of a leg. Because whether the motion is feasible or not depends on the angular velocity of the leg swing, we propose a method to calculate it approximately. This calculation is based on the phase plane analysis and on conservation of angular momentum. The main problem there is how to evaluate the motion during the rotation around hip, where angular momentum has an effect of the gravity. Here, we calculate it using the initial and final position of COG in the hip rotation phase. This method enables us to estimate a necessary angular velocity without solving dynamic equation. Finally, we examine the effectiveness of the method by simulation and experiments with our constructed robot.