The present invention provides a circular polarization illumination device which can irradiate right-circularly polarized light at a large light amount from a light source of right-circularly polarized light with a high efficiency and can irradiate left-circularly polarized light at a large light amount from a light source of left-circularly polarized light with a high efficiency, and a plant growth regulation method that can efficiently regulate the promotion or inhibition of the growth of plants by using the circular polarization illumination device. Moreover, the illumination device includes a light-emitting light source, a reflective polarizing plate, reirradiation unit, and circular polarization conversion unit.