A light source and a light irradiating device are provided to efficiently regulate a biological rhythm of a user without changing a color temperature of illumination light, an average color-matching index (Ra) or both. The light irradiating device is comprised of a blue light emitter (11) that emits light with major energy in a wavelength region adjacent to a wavelength to minimize a value of M(lambda)/z(lambda), where M(lambda) is a relative sensitivity in an antisecretory characteristic of melatonin and z(lambda)is a relative sensitivity of a color matching function, a blue light emitter (12) that emits light with major energy in a wavelength different from the wavelength of the blue light emitter (11), a red light emitter (13), a green light emitter (14), a diffusing plate (15) that diffuses light from each light emitter, a control device (16) that controls the light emitting intensity of each light emitter, an information input device (17) that inputs information on adjustment of the degree of biological reaction of each light emitter, information on an illumination color temperature and the like, a switch (18) that can turn on or off each light emitter, and a memory device (19) that stores the light emitting intensity of each light emitter and its adjusting methods.