A mono-layer liquid phase type water-based insecticidal aerosol comprises a base liquid for aerosol containing at least one specific pyrethroidal compound, a specific organic solvent and a specific buffer solution, which base liquid has a pH of from 7.0 to 11.0, and dimethyl ether as a propellant. The aerosol according to the present invention has a long-term storage stability as well as an excellent insecticidal activity and causes no degradation of the container.