We are planning to carry out a simulation to evaluate the safety level of an oceanic route system of the North Pacific. In this simulation a distribution model of inter-arrival time is necessary for generating traffic. Distributions of inter-arrival time for the west bound traffic were observed at the entrance fixes in the Tokyo FIR to characterize the traffic. Since the empirical distributions for each time period are different, a mixed distribution model representing the distribution of every one-hour time period is proposed. The model is composed of linear combination of each probability density function. This paper first describes the method and results of analyses, and then proposes the model. Finally, the validity of the model is demonstrated by generating pseudo traffic based on the model.