In this paper, novel force control method without force sensor is proposed. In general force control, force sensor such as strain gauge is used. But they have some drawbacks as represented by limited detectable point or narrow bandwidth. Using the proposed force error observer, sensorless force control and estimation of environmental force are realized. It uses only velocity information. It is found that force control results in a shift of inertia, which is named virtual inertia. The effectiveness of proposed controller is confirmed by simulation and experiment.