To provide a method whereby the amount of a phenolic compound such as polyphenol can be effectively/efficiently increased. The present invention provides a method for increasing the amount of a phenolic compound in a plant, said method being characterized by comprising irradiating the plant with ultraviolet light in such a manner that the irradiance of light within a wavelength range of 270-290 nm attains 1500-50000 μmol/m2 and the irradiance of light within a wavelength range of 310-400 nm attains a level corresponding to less than 50% of the irradiance of light within the wavelength range of 270-290 nm, or a method for producing a plant having an increased content of a phenolic compound.