The present invention provides a stabilized pharmaceutical composition that enables the production of a preparation for desensitization therapy useful as an agent for the prevention or treatment of allergic diseases such as pollen hypersensitivity, has outstanding allergen protein stability, and is useful for storage and transfer. More specifically, the present invention provides a stabilized pharmaceutical composition containing an allergen protein and at least one type of stabilizer selected from the group consisting of sugars, sugar alcohols, and sugar fatty acids.