For probabilistic safety assessment of ship it is necessary to estimate collision probability of the assessed ship. To estimate the probability first the encounter probability with other ships at dangerous situation must be estimated. In this paper analytical method to estimate encounter probability from sailing ship density is introduced and the correctness of the method is validated by simulation. This method is composed of 2 stages. First stage is to derive the probability of sailing ship density from the probability of appearing ships at the border of the gaming area. From the result of this stage the parameter λ (mean ship appearing ratio at the border) is related to ρ (the sailing ship density).Second stage is to derive the encounter probability with other ships from λ. At this stage the encounter probability is related to ρ. The fitness of the probabilistic density function obtained by this method are checked by χ² test using the results of the ship sailing simulation and this method is proved to be good estimation method of encounter probability.