Discharges of zinc, arsenic, cadmium, and lead from farmland were examined based on field measurements conducted between June 15 and July 13, 2006. The study fields are located in a lowland area where the irrigation water was contaminated with water drained from upstream farmlands (cyclic irrigation). The area of the farmland is 11.2 ha, of which 7.6 ha and 3.6 ha had been used for rice and soybeans, respectively. Heavy metal loads accompanying the rainfall runoff were considered to become higher with the increase in rainfall. Seventy percent of the zinc load, 62% of the arsenic load, 70% of the cadmium load, and 69% of the lead load were discharged with the rainfall runoff even though the rainfall runoff time was only 21% of the observation period.