A luminous device includes one or more line-shaped luminous units each including an elongated light guide, one or more light sources, and a reflective layer. The light guide has a generally flat luminous surface for emitting light and another surface disposed away from the luminous surface. The light source serves to provide light into the light guide. The reflective layer is disposed on the above-mentioned another surface of the light guide. The light guide may have a ring shape. Since the luminous device produces a continuous line-shaped luminous image, e.g., a ring-shaped luminous image, it may be used for clearly indicating a heating range of an induction heating cooker.