A method according to the present invention for producing a useful protein by using a plant includes a step (cultivation step) for cultivating a plant, a step (infection step) for infecting the cultivated plant with an agrobacterium having a polynucleotide for coding the useful protein, and a step (expression step) for causing the infected plant to express the useful protein. Furthermore, the method is characterized in that during at least part of the cultivation step, the plant is cultivated under irradiation conditions in which the light energy in the 600-700nm wavelength region constitutes 50% or more of the light energy in the 400-800nm wavelength region.
