Agricultural land has a function, such as a reservoir, that temporarily stores up storm flow during floods. This flood storage effect of agricultural land, which is known as the flood mitigation function, is one of the many important functions for country conservation which have been working from the very beginning days of agriculture. In this paper, the flood mitigation function of agricultural land in a flat low-lying area with creek networks has been investigated quantitatively. First, three virtual basin models were constructed by selecting the typical characteristics of the flat low-lying area in the Saga plain facing the Ariake Sea: paddy field basin model, upland field basin model and urbanized basin model. Second, the difference of flood runoff characteristics caused by the land utilization, the creek area ratio, and the existence of optimal control of drainage facilities, was evaluated using numerical simulations from the viewpoint of flood mitigation. Finally, an indicator to estimate the flood mitigation function of agricultural land in a flat low-lying area was studied in a macro-view. It was concluded that the flood mitigation function of agricultural land in a flat low-lying area with creek networks could be evaluated by the macro-indicator MI, consisting of three sub-indicators, i .e. I_α for the land utilization, I_β for the creek area ratio, and I_γ for the optimal control of drainage facilities. It was also suggested that the fuzzy reasoning　was an effective technique for the optimal control of the drainage system in a flat low-lying area with creek networks.