When a human needs to apply a large force to an environment, the human will take an appropriate posture to produce a large manipulation force. This paper discusses appropriate working postures for a humanoid robot to apply a force effectively to an environment. An objective function is defined and the Sequential Quadratic Programming (SQP) is used to find a solution. The pushing a wall and turning a valve are taken as examples of tasks for a humanoid robot, and simulations and experimentations are performed. The results show that the consideration of working postures clearly contribute to make the manipulation force larger.