In this paper, we proposed a soft skin design method of human symbiotic robots which achieves collision safety in all collision situations. At the same time, soft skin design has to be optimized for the collision safety and the workability. Therefore we evaluate effectiveness of soft skin at each position on arm in longer-direction, and optimize its thickness along collision position. Firstly we discuss parameters for describing the collision between human and human-symbiotic robots, and identify the worst case collision including the out-of-control condition of the robots. Next we conducted actual collision experiments duplicating the worst case collision and apply the proposed method to soft skin covering of human-symbiotic robot "TWENDY-ONE". In consequence, TWENDY-ONE acquires both of collision safety and the workability.