Abstract: An illumination device includes a light source 45, a reflection plate 41 for reflecting the light irradiated from the light source 45, and a transmissive and diffusive member 42 for transmitting and diffusing the light from the light source 45 and the reflected light from the reflection plate 41, so that the light transmitted and diffused in the member 42 may be used for illumination. With this indirect illuminating system, an entire area can be uniformly illuminated with high efficiency, almost free of luminance spots. When the illuminating device is used in a plant growth apparatus, uniformity of plant growth is enhanced and the yield of the plants is improved. In addition, space for planting can be utilized to the maximum so that the number of grown plants significantly increases as compared with the conventional direct illumination system.