An illumination module is provided in the invention. The illumination module includes a light source having a plurality of light emitting elements arranged in a linear array and a light guide device. The light guide device receives light from the light source and generates uniform light which is transmitted onto an article to be scanned, has a longitudinal direction, and includes a diffraction structure and a reflection portion. The diffraction structure has a shape inducing light to be scattered and deflects the scattered light toward the longitudinal direction. The reflection portion reflects the light scattered and deflected by the diffraction structure, and is inclined to have an inclined angle with the light propagating direction. In addition, the reflection portion is formed on the diffraction structure or apart from it.