We developed a simulation model to predict the seasonal variation of biomass (wet weight) of Metridia pacifica in Toyama Bay, the southern Japan Sea, by a combination of the two modified submodels: growth and individual number ones. The former submodel was due to the relationship between development and growth of this species and the mean temperature of their habitat in Toyama Bay. The latter submodel was based on the experimental data on the survival rate of Acartia clausi (Copepoda:Calanoida) from literature. The present model consistently simulated the annual maximum of M. paciflca biomass in the beginning of April, as indicated by the field observations, by reducing the survival rate used for A. clausi by half during the copepodite V and VI (adult) stages under the effect of a possible fish (ex. mackerel and fire-y squid) predation.