Disclosed is an illuminating device which, as a light source for a backlight device, mixes the colors of light rays of three prime colors radiated by light emitting diodes to emit white light. The illuminating device includes a first light source (22R) radiating light rays of the first prime color, a second light source (22G) radiating light rays of the second prime color, a third light source (22B) radiating light rays of the third prime color, optical units (23R), (23G) and (23B) which refract divergent light rays contained in the light rays of the first prime color, emitted by the first light source (22R), the light rays of the second prime color, emitted by the second light source (22G) and in the light rays of the third prime color, emitted by the third light source (22B), to form collimated light rays, and triangular prisms (24) and (25) or a dichroic prism (26) mixing the light rays of the first, second and third prime colors, emitted by these optical units, by selective transmission and reflection, based on optical properties of the light rays of the respective prime colors.