The present invention provides an illuminating device that makes it possible to reduce the number of members for regulating a polarization state and can irradiate light having natural color shades without decreasing energy efficiency of light irradiation, and a plant growth regulation method using the illuminating device. The illuminating device of the present invention includes a light-emitting light source and a polarization state regulation member that regulates a polarization state of the light-emitting light source, in which the polarization state of a wavelength region of a portion of emission wavelengths is changed to circular polarization, and a degree of circular polarization of light in a wavelength band for regulation among the light rays to be irradiated is 0.3 or higher. In a preferable embodiment, a width of at least one wavelength band for regulation of the polarization state regulation member is from 60 nm to 250 nm.