With the international trade, worldwide demand for overhead cranes is increasing. It is difficult to operate cranes and expert skill is needed due to their underactuated system. This is one of the biggest problems. However, this problem can be solved by automatic control and assist control. Operators are not needed in automatic control and it can reduce the cost and the risk of accidents due to negligence of operators. Assist control can help operators to operate crane and it also can prevent accidents. Many researches on automatic control and assist control have been already done. However, there is another problem. The problem is that the sway of a payload cannot be measured accurately with a sensor. This problem can decrease the efficiency of operators. In this paper, we propose a new method to measure the sway angle of the payload without the sensor. Using the reaction force observer, the power of the motor to suspend the payload with a rope can be estimated and we can estimate the sway angle. The efficiency of this method is verified by simulations and experiments.