PROBLEM TO BE SOLVED: To provide an illumination device which can reduce the number of members that control a polarization state and enables light irradiation of a natural hue without lowering the energy efficiency of the light irradiation, and to provide a method for controlling the growth of a plant using the illumination device.SOLUTION: In the illumination device which includes a light-emitting light source 3 and a polarization state control member 4 for controlling the polarization state of the light-emitting light source, the polarization state in a wavelength region of a part of the light-emitting wavelength is changed to circular polarization, the degree of the circular polarization of the light in a control wavelength band out of the light to be emitted is 0.3 or more, and at least one control wavelength band of the polarization state control member is 60 nm or more and 250 nm or less.【課題】偏光状態を制御する部材の点数を少なくでき、光照射のエネルギー効率を下げることなく自然な色合いの光照射が可能な照明装置、及び該照明装置を用いた植物の成長制御方法の提供。【解決手段】発光光源３と、該発光光源の偏光状態を制御する偏光状態制御部材４とを備えてなり、発光波長の一部の波長領域の偏光状態を円偏光に変更し、照射される光のうち制御波長帯域における光の円偏光度が０．３以上である照明装置。該偏光状態制御部材の少なくとも１つの制御波長帯域幅が６０ｎｍ以上２５０ｎｍ以下である照明装置。【選択図】図１
