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 amounts of nitrogen, phosphorus and potassium as fertilizer ingredients as used at the step (B) are smaller than amounts of nitrogen, phosphorus and potassium as fertilizer ingredients, respectively, as used at the step (A). Preferably, fertilizer ingredients are applied in amounts such that a growth medium at the step (B) contains 10-15 me/L of nitrogen, 1-4 me/L of phosphorus and 2-6 me/L of potassium, and a growth medium at the step (A) contains 15-20 me/L of nitrogen, 3-6 me/L of phosphorus and 6-9 me/L of potassium.