To induce the expression of human clock genes.SOLUTION: Provided is a light irradiation device 1 for irradiating human skin cells with the light, comprising a light source unit 12 for emitting light, the light source unit 12 emitting a first light H1 having a wavelength shorter than a wavelength of 505 nm and/or a second light H2 having a wavelength longer than a wavelength of 635 nm, among a wavelength range from visible light to near infrared light, and the clock gene being expressed by irradiating the skin cells with the light.SELECTED DRAWING: Figure 7【課題】ヒトの時計遺伝子の発現を誘発すること。【解決手段】光を出射する光源部１２を備え、前記光をヒトの皮膚細胞に照射する光照射装置１であって、前記光源部１２は、可視光から近赤外光までの波長範囲のうち、波長５０５ｎｍよりも短い波長の第１の光Ｈ１、及びまたは、波長６３５ｎｍよりも長い波長の第２の光Ｈ２、を出射し、前記光が前記皮膚細胞に照射されることで時計遺伝子を発現させる。【選択図】図７
