Our research group has developed the small weeding robot inspired from rice duck farming. This robot can work such as rice duck farming by rotating rod wheel which this robot has. In order to work in the rice field such as loose soil, we have also developed this wheel. An ultimate goal of our research group is to control the robot automatically. For achieving automatic control, we need to model driving force mechanism of rod wheel on loose soil. The robot can work suitable automatic control by modeling driving force mechanism. The rod wheel obtains driving force by digging soil. Therefore, driving force from this wheel depends on state of soil. In this study, we model driving force mechanism for rod wheel on loose soil.