In recent years, collision safety has been one of the most important issues for service robots. To ensure collision safety assurance, a passive compliance method is preferred to an active one because it can provide faster and more reliable responses to dynamic collision. Since both positioning accuracy and collision safety are equally important, a robot arm should have very low stiffness when subjected to a collision force greater than the one causing human injury, but maintain very high stiffness otherwise. In order to realize these ideal features, a novel safe joint mechanism (SJM) composed of linear springs and a modified slider-crank mechanism is proposed in this paper. The SJM has the advantages of variable stiffness which can be achieved only by passive mechanical elements. Various experiments on static and dynamic collisions show the high stiffness of the SJM against an external force of less than the critical impact force, but an abrupt drop in the stiffness when the external force exceeds this critical force, which guarantees positioning accuracy and collision safety. Furthermore, the critical impact force can be set to any value depending on the application and the environment.