There is a problem that Microcystis bloom often occurs in the Tuchiura-bay, Kasumigaura Lake. Some previous studies indicate there are three processes to occur Microcystis bloom; transportation by the wind; recruitement from the sediment; the vertical miguration, however affection of these processes on Microcysts bloom in the Tuchiura-bay is uncrear. We constructed the ecosystem simulation model to examine the influence of these processes the Microcystis bloom. As the results, the transportation process effected Microcysts bloom because the data by this simulation model added transportation process similar to data of actual decrease of the cell density by diffusion. Meanwhile this model expressived locally and temporary high density of Microcystis cells. It is suggested that the recruitment process has influence on Microcystis bloom, but we need more study with the annual detail data of recruitement from sediment. It is suggested that the vertical migration process has less effect on Microcystis bloom than others.