This paper proposes a sensor system for mobile robot for rough terrain. Our sensor system consists of 2 active joints and 1 distance measuring sensor. This distance measuring sensor is general purpose type distance measuring sensors which consist of PSD (Position Sensitive Detector) and infrared emitting diode and signal processing circuit. The main superior points are as follows: 1) this system is compact and light weight for enough attaching to small mobile robot, 2) It is very simple structure; which enables to do easy maintenance, 3) Since the sensor is attached to the robot, the sensor system should make full use of robot's degree of freedom (DOF), namely that's an active sensing. This paper explains the system structure of the proposed sensor system, and basic experimental result to confirm performance of this system.