The authors propose force control strategies that are robust against disturbance and parameter variation. These strategies are expansions of a disturbance observer which estimates disturbance and parameter variations of motors with a simple computation. Observers are proposed to estimate parameter variations of the environment on which the force is imposed. Since the control system is fixed to a nominal system by the observer, a second derivative of force can be controlled. Force control is introduced using a controller of the second derivative of the force. The force response similar to the force command is realized in spite of the disturbance and the parameter variations of the control object. The effectiveness was confirmed experimentally.