This paper proposes a force control of a two-mass resonant system based on modal transformation. In the conventional force-control method in resonant system, the control goals are not clearly defined from the point of the view of control stiffness. Moreover, there is a problem that it is needed to know value of an environmental stiffness for setting force gain. Therefore, in this paper, the control stiffness in the case of force control of the two-mass resonant system is defined on the modal spaces which are composed of rigid and torsional modes. Concretely speaking, in the force control, it is defined that a control stiffness of a rigid mode should be zero. Based on the control stiffness, a force control of the two-mass resonant system is designed. Vibration suppression is conducted in torsional mode. On the other hand, in rigid mode, force control is conducted. The validity of the proposed method is verified by both simulation and experimental results.