This work presents a new practical environment modeling for mobile robots in an unknown circumstance. To perform environment modeling, obstacles located in the circumstance should be detected using sensor data. Then some sensor fusion methods are used to model environment better. Environment modeling shown In this work is based on the heuristic sensor fusion method. In this method, we define a segment as one ultrasonic sensor and two adjacent infrared sensors. Especially, infrared sensors are used to reduce defects of ultrasonic sensor such as poor directionality and frequent misreading. Whether infrared sensors of a segment detect obstacles or not, ultrasonic sensor data are modified. To evaluate the usefulness of the sensor fusion method, we used semicircular sensor module composed of 6 ultrasonic sensors and 7 infrared sensors. With this module, several experiments in indoor environment are performed.