As a plant cultivation method by an artificial light irradiation which is convenient, highly energy efficient, and excellent in growth promoting effect, a plant cultivation method which promotes the plant growth by conducting a step S1 for irradiating a red illuminative light to a plant and a step S2 for irradiating a blue illuminative light to the plant separately and independently of each other within a certain time period. In this plant cultivation method, an extremely remarkable plant growth promoting effect can be obtained by a method as simple as an alternate irradiation with a red illuminative light and a blue illuminative light.