The application of a model of long-term runoff of herbicide mefenaset from paddy fields to a river was investigated. The model of long-term runoff of mefenaset to the river was constructed as follows. The change in the concentration of mefenaset in ponding water provided a pseudo-first-order rate, and a flow model provided the amount of the surface runoff in the paddy field of the tank model. For the model of long-term runoff, although there were little gaps, the characteristic of mefenaset from paddy fields of the runoff to a river was reproduced. From the simulation results, it is considered that it is possible to use it for a sufficiently precise forecast of the movement of applied pesticides in the environment and use it for runoff load reduction.