An illuminating device that can reduce illuminance unevenness of an irradiation target face with a simple construction is provided. In a pseudo sunlight illuminating device 1 having a lamp 22 for illuminating an irradiation target face 10A, a transmission light amount adjusting unit 60 for adjusting a transmission light amount so that the illuminance distribution on the irradiation target face 10a is made uniform is provided between the lamp 22 and the irradiation target face 10A, and the transmission light amount adjusting unit 60 is provided with a light transmission plate laminate member 64 which is constructed by stacking light transmission plates 65 which are constant in transmittance in a wavelength range of light to be transmitted and whose number corresponds to an adjustment amount of the transmission light amount so that incident light is reflected at each boundary of the front and back surfaces of each light transmission plate 65.