An Integrated Navigation System (I. N. S.) is expected to improve both the safety of navigation and the operation efficiency. The authors have proposed the support system for collision avoidance in I. N. S. and also developed the collision avoidance expert system which proposes the collision avoidance action considering the prediction of target ship's action, traffic rules and navigator's practices. The authors reported on these studies in previous papers. In this paper, the authors report the results of some experiments on the intelligent support of decision making in I. N. S. that were conducted at the ship maneuvering simulator equipped with I. N. S. and collision avoidance expert system. The conclusions are as follows : (1) The analysis of the situation is improved when navigator uses the prediction of the target ship action provided by the expert system. It is especially efficient during multi-encounter situation in congested water and encounter situation in curved channel. (2) The combination of the safety evaluation method used in I. N. S. and the collision avoidance expert system assures proper decision-making and reduce the complexity of the decision-making during the collision avoidance. (3) The investigation shows that the improvement of the safety of navigation will follow the development of the intelligent decision-making support system.