The use of a medium-chain fatty acid triglyceride as the dispersant in the preparation of a medicinal aerosol composition comprising tricyclic compound (I) dispersed in a liquefied hydrofluoroalkane propellant is described. When a liquefied hydrofluoroalkane is added to a kneaded premix of the tricyclic compound (I) and a medium-chain fatty acid triglyceride, the active ingredient is evenly dispersed in the liquefied hydrofluoroalkane. Therefore, by distributing a dispenser first with the kneaded premix and, then, with a liquefied hydrofluoroalkane under cooling or elevated pressure, there can be provided a medicinal aerosol composition having an improved uniformity of content of the active ingredient.