The present invention configures an alternating dark period and a light period during the rest period of a plant. During the dark period, the intensity of light emitted at a plant being cultivated is less than the light intensity at the compensation point. During the light period, blue light of a wavelength of 400-500 nm having a lower light intensity than the light intensity at the compensation point is emitted. The time T of each cycle of repetition of the dark period and light period is 2-500 µs. The duty cycle (ΔT/T) of the time ΔT of the light period to the time T of each cycle of repetition is not more than 20%. The photosynthetic photon flux density of the blue light is 0.001-4.0 µmol·m-2s-1.
