The present invention provides a corona-core microgel emulsifying agent composed of a copolymer obtained by polymerizing polyethylene oxide macromonomers, hydrophobic monomers, and cross-linking monomers under specific conditions as well as an oil-in-water emulsified composition characteristically using said emulsifying agent for emulsification.The object of the present invention is to provide a new corona-core microgel emulsifying agent to be used to prepare an oil-in-water emulsified composition that manifests superior emulsification stability, stability over time, and low skin irritation, and is free of stickiness at the time of application, manifests dewy freshness, is free of powdery sensation and/or squeakiness, and is superior in durability of fragrance, as well as an oil-in-water emulsified composition emulsified with said emulsifying agent.