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 humidity in a cultivation atmosphere at the step (A) is higher than that at the step (B). Preferably the humidities in a cultivation atmosphere at the step (A) and the step (B) are in the ranges of 60%-90% and 40%-60%, respectively.