For safer traffic environment in a congested bay, introducing a major marine roundabout is an effective countermeasure, which reduces the number of unsafe crossing encounters. However, No sufficient understanding has been obtained about the traffic behavior of ships on a major roundabout. Under this background, the authors proposed a mathematical model to clarify the effects of the roundabout on the traffic phenomena. This paper extends the model by introducing the distribution of ship size of the traffic to relax the unrealistic assumption that ship size is unique for all the ships in the model. The effects and the usefulness of taking the ship size distribution into analysis is demonstrated through numerical examples.