By adding the cost conception to the results of the queuing theory, diagrams evaluating optimum number of berths in ports and harbours are presented.
　In using the diagram, it is assumed that the arrival distribution is Poisson distribution and service distribution is exponential, Erlung Distribution with phase 2 or 3, or the regular distribution. Parameters needed for the diagram are λ：average arrival rate, μ: average service rate, cb: berth idling cost per unit time if empty, and cw: ship waiting cost per unit time.
　If c'b: total cost of construction and maintaince during the period T per berth, is taken instead of cb, the optimum number of berths can be obtained by the same diagram but with the slightly defferent procedure.