Unlike industrial manipulators, the manipulators mounted on service robots are interacting with humans in various aspects. Therefore, safety has been one of the most important design issues. Many compliant robot arms have been introduced for safety. It is known that passive compliance method has faster response and higher reliability than active ones. In this paper, a new safety mechanism based on passive compliance is proposed. Passive mechanical elements, specifically transmission angle of the 4-bar linkage, springs and shock absorbing modules are incorporated into this safety mechanism. This mechanism works only when the robot arm exerts contact force much more than the human pain tolerance. Validity of this mechanism is verified by simulations and experiments. It is shown that the manipulator using this mechanism provides higher performance and safety than those using other passive compliance mechanisms or active methods.