In order to search and rescue the victims in rubble effectively, a 3D map of the rubble is required. As a part of the national project of rescue robot system, we are investigating a method for constructing a 3D map of rubble by teleoperated mobile robots. In this paper, we developed a laser range finder for 3D map-building in rubble. The developed range finder consists of a ring laser beam module and an omnivision camera. Ring laser beam is generated by using a conical mirror and it is radiated toward interior wall of the rubble around a mobile robot that mounts the laser range finder. The omnivision camera with hyperbolic mirror can capture the reflected image of the ring laser on the rubble. Based on the triangulation principle, a cross section range data is obtained. Continuing this measurement as the mobile robot moves inside the rubble, a 3D map is obtained. We constructed a geometric model of the laser range finder for error analysis and obtained an optimal dimension of the laser range finder. Based on this analysis, we actually prototyped a range finder. Experimental results show that the actual measurement errors are well matched to the theoretical values. Using the prototyped laser range finder, a 3D map of rubble was actually built with reasonable accuracy.