The motion behavior of a skid-steering rover on loose terrain is modeled experimentally, and a position estimation method based on the model is proposed. Skid steering is a steering method adopted for various vehicles and robots. In this method, the vehicles steer by controlling a ratio between the rotational velocities of their left and right wheels. For controlling a skid-steering rover, whose wheels can slip on the lunar and planetary surfaces, which are covered with loose regolith, it is important to model accurately the rover's motion behaviors on loose terrain. To this end, we conducted traveling tests using a four-wheeled rover in a sand field and analyzed the motion behaviors of the rover on loose terrain. In the tests, the velocities of the rover's left and right wheels were controlled, and the rover traveled with various turning patterns. From the test datasets, we derived a mathematical model that expresses a relationship between the input and output velocities of the rover's wheels. Furthermore, the effectiveness of a position and motion estimation method based on the model was confirmed from experiments conducted in the sand field.