To improve navigation safety and to reduce operation cost, it is very important to develop an integrated navigation system (INS). Such an INS should have a function to help collision avoidance maneuvers for a human operator and the authors have tried to design such a system and to evaluate its effectiveness. First, a prototype of INS was designed taking roughly human's decision-making process into consideration. The system was set in the ship handling simulators of Hiroshima University and Yusen Marine Science Inc. and simulation studies were carried out in order to improve and evaluate the system. Many kinds of supporting functions have been tested and finally several functions have been proposed for assisting decision-making and command-execution of navigators for collision avoidance. The conclusions in the present study are as follows : (1) A so-called proto-typing procedure that a proper prototype system is first designed and then it is improved by means of real-time navigation simulations is effective for the development of such a system to support human operators. (2) An automated position-leading function is very important for making a human operator free from load to decide timing of maneuvers as well as for achieving precise control. (3) To realize effective supports in INS for collision avoidance is necessary to take human's decision-making process into consideration, and some desirable methods are finally proposed from such viewpoints.