A human-robot coexisting system requires the essential function such as safety and flexibility which are not common in general industrial robots. To build up such a robot system, an inherently flexible actuator must be effectively used rather than a conventional rigid actuator. A pneumatic actuator seems just available as such a human friendly actuator. In this paper, a property of the flexibility of pneumatic actuator is analyzed compared with a DC electric motor. Also the application of a pneumatic rubber artificial muscle actuator to a rehabilitation robot is discussed. The results show that a pneumatic actuator can well work as one of human friendly actuators with cooperation of a proper control strategy.