A method for producing a microcapsule, comprising: heating an aqueous dispersion comprising: a dispersion stabilizer; an anionic surfactant; an organic solid biologically active substance that is selected from flumioxazin and 1-(2-chloro-6-propylimidazo[1,2-b] pyridazin-3-ylsulfonyl)-3-(4,6-dimethoxy-pyrimidin-2-yl)urea; a polymerization initiator; and a radical-polymerizable monomer; wherein the aqueous dispersion has been subjected to ultrasonic irradiation and wherein the concentration of the anionic surfactant is lower than its critical micelle concentration; and specifically wherein the method comprises: (a) preparing a mixture solution by dissolving the dispersion stabilizer and the anionic surfactant in water, dispersing the solid biologically active substance in the water, and adding the polymerization initiator and the radical-polymerizable monomer to the water; subjecting the mixture to ultrasonic irradiation and stirring comprising (i) performing ultrasonic irradiation for a period of 1 minute to 30 minutes, thereby reducing the volume median diameter of the radical-polymerizable monomer to 2 μm or less, and (ii) then continuing stirring for 30 minutes to 5 hours; and then heating the mixture solution at a temperature not lower than the reaction temperature of the polymerization initiator, thereby producing the microcapsule; or (b) mixing a liquid having an aqueous phase containing the dispersion stabilizer uniformly dissolved therein and the pulverized solid biologically active substance dispersed therein with a liquid prepared by adding the radical-polymerizable monomer containing the polymerization initiator dissolved therein to an aqueous phase containing the anionic surfactant and the dispersion stabilizer, which are uniformly dissolved therein; and subjecting the mixture to ultrasonic irradiation and stirring comprising (i) performing ultrasonic irradiation for a period of 1 minute to 30 minutes, thereby reducing the volume median diameter of the radical-polymerizable monomer to 2 μm or less, and (ii) then continuing stirring for 30 minutes to 5 hours; and then heating the mixture solution at a temperature not lower than the reaction temperature of the polymerization initiator, thereby producing the microcapsule. (a) preparing a mixture solution by dissolving the dispersion stabilizer and the anionic surfactant in water, dispersing the solid biologically active substance in the water, and adding the polymerization initiator and the radical-polymerizable monomer to the water; subjecting the mixture to ultrasonic irradiation and stirring comprising (i) performing ultrasonic irradiation for a period of 1 minute to 30 minutes, thereby reducing the volume median diameter of the radical-polymerizable monomer to 2 μm or less, and (ii) then continuing stirring for 30 minutes to 5 hours; and then heating the mixture solution at a temperature not lower than the reaction temperature of the polymerization initiator, thereby producing the microcapsule; or (b) mixing a liquid having an aqueous phase containing the dispersion stabilizer uniformly dissolved therein and the pulverized solid biologically active substance dispersed therein with a liquid prepared by adding the radical-polymerizable monomer containing the polymerization initiator dissolved therein to an aqueous phase containing the anionic surfactant and the dispersion stabilizer, which are uniformly dissolved therein; and subjecting the mixture to ultrasonic irradiation and stirring comprising (i) performing ultrasonic irradiation for a period of 1 minute to 30 minutes, thereby reducing the volume median diameter of the radical-polymerizable monomer to 2 μm or less, and (ii) then continuing stirring for 30 minutes to 5 hours; and then heating the mixture solution at a temperature not lower than the reaction temperature of the polymerization initiator, thereby producing the microcapsule.