An illuminating device comprising a light source for emitting a plurality of light beams (207, 208, 209) of different colors, a plurality of rotating beam scanners (102, 103, 104) and a plurality of scanning lenses (109, 110, 111). The beam scanners have spiral reflection surfaces (102, 103, 204) formed on the outer peripheries of cylindrical bodies, Each colored light beam is shone onto each reflection surface from a direction parallel to a rotating axis, and magnified by each scanning lens after reflected to scan an illuminated area with a plurality of beams of different colors one after another. Accordingly, the illuminating device can enhance scanning linearity, reduce noise due to a minimal windage loss, and decrease costs and power consumption.