In practice, a lot of manufacturing tasks performed by robots involve extensive contact with objects in workplace. In such tasks it is required to control the forces of interaction between the manipulator and the objects directly. Various force control algorithms have been developed which can be based on two distinct methodologies: hybrid force/position control and impedance control. In order to apply the force control algorithms, force sensors must be used. However, force sensors have drawbacks that make them not suitable in some applications. The purpose of this paper is to present control algorithm for interactional force between an end-effector of a nDOF robot manipulator with a rigid object in which a state observer has been used to replace force sensors.