There are various factors to be taken into account for the objective safety evaluation and comprehensive performance evaluation of an automatic ship navigation system. A computer simulation is an effective tool for the safety assessment. A new computer system suitable for the simulation of the ship navigation and marine traffic is designed and built. The simulation system is composed of a network of different kinds of computers one is for numerical calculation using FORTRAN, the second for knowledge processing and the others are graphic work-stations. Realistic representation of environmental conditions, such as natural, geographical and vessel traffic condition, and suitable modeling of ship motion and judgment and decision-making for ship operation are essential for the evaluation. Simulation monitors by CGI technique are also effective for the subjective evaluation of the simulation results. Composition and features of the simulation system are introduced here. The real time simulation system with multi-process type is driven by simulation scenario, which contains various for the evaluation. The simulation of an automatic ship navigation in Tokyo Bay shows satisfactory results. The system is expected as a powerful tool for the concrete safety evaluation of the navigation system.