The invention provides a method for controlling weeds in wet - rice cultivation which comprises: a step of spraying a water-diluted solution comprising a herbicide such as 1-(2-Chloro-6-propylimidazo[1, 2-b]pyridazin-3-ylsulfonyl)-3-(4, 6-dimethoxypyrimidin-2-yl)urea onto paddy fields and a step of submerging the paddy fields after the above treatment with the herbicide.