Abstract: A one-phase microemulsion composition, which can be easily prepared without the use of special equipment, has high safety, and stays stable for a long period in an ultrafine emulsion state when the composition is added to an aqueous formulation, is provided by appropriately adjusting the ratio of (A) a hydrophilic nonionic surfactant, (B) a lipophilic nonionic surfactant, (C) an oil component, (D) a water-miscible solvent that does not interdissolve with the oil component, and the critical micelle concentration (CMC) of the hydrophilic nonionic surfactant in the water-miscible solvent is higher than that of the hydrophilic nonionic surfactant in water, and (E) water.