A plant-cultivating method is provided which comprises a red light irradiation step (A) and a blue light irradiation step (B), wherein the step (A) and the step (B) are independently carried out for a predetermined period of time under cultivation conditions such that the concentration of carbon dioxide in a cultivation atmosphere at the step (B) is higher than that at the step (A). Preferably the concentrations of carbon dioxide at the step (B) and the step (A) are 1000-2500 ppm and 700-1500 ppm, respectively. The concentration of carbon dioxide at the step (B) is preferably at least 200 ppm higher than that at the step (A).
