This paper presents a 3D sensing system by a tracked vehicle robot using a laser range finder (LRF) with an arm-type movable unit. The unit is mounted on the robot and the LRF is installed at the end of the unit. This sensing system enables the sensor to change position and to face at a right angle corresponding to a variety of configuration. Because the occlusion can be avoided by this mechanism even for complex terrain, the robot can measure a 3D configuration such as valley, gap, upward or downward stairs more accurately than conventional 3D sensing system with the LRF, which have generally used a rotating mechanism. In this study, we have designed and developed a prototype system of the movable LRF unit and mounted it on a tracked vehicle robot with two crawlers that we have been developing. Experimental results for basic 3D sensing showed that proposed system is useful for sensing and mapping more complex terrain.