The safety of the North Pacific route system has been evaluated using a collision risk model. Passing frequency is one of the important parameters of the model. The passing frequency is the twice of expected frequency of overlapping event in the longitudinal direction. It depends on the route structure, the number of flight and so on. In this paper, monthly lateral and vertical passing frequency values for the North Pacific route are calculated using flight plan data of about eight years and long-term transition of the passing frequency values are investigated. One result obtained shows that the passing frequency values under the implementation of a reduced vertical separation minimum do neither increase nor decrease remarkably.