Because natural frequency of the rope-load oscillation system affects the stability and performance of the control system, this paper proposes a simple robust controller to achieve this problem. Firstly, a linear dynamic model of a rotary crane is derived by using a disturbance observer. The model has robustness with respect to varying parameters such as joint friction, load mass and velocity of horizontal boom motion. Next, a state feedback controller with integrator is designed based on the model, and controller gains are determined by using linear matrix inequality (LMI) optimization for achieving robustness with respect to rope length variance. Finally, comparative simulations and experimental results demonstrate the effectiveness of the proposed method. Therefore, the crane can be easily operated without sensor systems for measuring rope length, consequently, the structure of the crane can be simplified and implementation cost can be reduced.