A motion control method of lifting a heavy object up to a higher position by humanoid robots is developed. Key issue of lifting motion is how to reduce the load on humanoid arms in which lowed power actuators are implemented. The use of singular postures of arms is well-known to avoid actuator saturation of the arms. We propose a motion planner based on optimization method with making better use of actuator’s characteristics. By combining two different kinds of humanoid motions such as accelerating an object upward and sliding the body into under the object, we show that the method enables to transit one singular posture of arms to another while lifting the object. We verify the effectiveness of the proposed method for reducing the load on the arms through simulation and experiment.