A mechanical system, such as a robot system for man-robot cooperation, a master-slave manipulator system, a power-steering system, an extender etc. has interactions among the system, the operator and its environment. How to control the interactions so that the desired interaction is realized, is one of the key issues for the control of such a mechanical system. The design of the controller of mechanical systems with man-machine interactions is discussed in this paper. We first model the mechanical system, which has interactions with its environment and an operator, then propose a control algorithm so that the desired interaction is realized. The proposed control system, consists of two controllers: one generates the desired motion of the mechanical system based on the force applied to the system, and the other one controls the interaction between the system and its environment The proposed control algorithm specifies both force augmentation and maneuverability of the system. The algorithm is applied to a planar manipulator with one degree of freedom and the experimental system illustrates the concept of the system.