PROBLEM TO BE SOLVED: To provide a method and device for increasing yield of plant polyphenol enabling sufficient promotion of synthesis of polyphenol to prevent defective coloration or shortage in antioxidative components. SOLUTION: The device for increasing yield of polyphenol 1 is provided with a white lamp 2 emitting visible light, and a fluorescent light 3 emitting UV-B light. The fluorescent light 3 has a borosilicate glass cylinder 6 eliminating a wavelength of 280-300 nm and a wavelength of 340-550 nm. A U-shaped cultivation container 5 is disposed directly under the white lamp 2 and the fluorescent light 3. Culture media 7 is charged in the cultivation container 5, and, and baby leaves 4 are planted in the culture media 7. The baby leaves 4 are irradiated with the white lamp 2 having 150-600μmol/m2/sec and the fluorescent light 3 having≥100μW/cm2. The UV-B light comprises the one from which the wave length of 280-300 nm and 340-550 nm falling in the fluorescent light 3 are eliminated. COPYRIGHT: (C)2007,JPO&INPIT