A small AUV "MR-X1" is being developed for detailed investigations of the seafloor and marine organisms at JAMSTEC (Japan Agency for Marine-Earth Science and Technology). The motion of the vehicle is controlled by five thrusters. In order to improve control performance, digital implementation of a model based controller (LQI controller) was begun in 2009, and field experiments were carried out in the sea area near the volcano "Teishi Kaikyuu" in Sagami Bay on February 8, 2010. In this paper, the results of field experiments on direction control of the vehicle are summarized, and the motion equations of the vehicle required for a model based controller (LQI controller) are modified by using the results of the field experiments.