To achieve force control of an industrial robot, this paper proposes a new force control method based on the spring ratio and the instantaneous state observer. To analyze the behavior of an industrial robot in contact with the environment, this paper analyzes a two-inertia system in contact with the environment. On the basis of the resonance ratio considering the environment, this paper shows that the stability of the resonance ratio control depends on the bandwidth of the torsional torque estimation. To achieve stable resonance ratio control, this paper employs the resonance ratio control with the instantaneous state observer. In addition, a force control system using the I-P force controller and the instantaneous state observer is employed. This paper shows that the resonance ratio of the force control system is determined to be the spring ratio S. The effectiveness of the proposed method is confirmed by a numerical simulation and experiments using the industrial robot arm.