An LED light source device and an illumination device using the same, including vehicle lighting devices, can have a light weight and thin profile and can form a desired light distribution pattern while employing a simple configuration. The light source device can include a plate shaped light guide member made of a material that is transparent in a visible range. The light guide member can have a front surface serving as a light emitting surface and having a plate shape, and a rear surface having a luminance control element for controlling a luminance distribution on the light emitting surface. A point or linear light source facing towards an end surface of the light guide member can be provided. The luminance control element controls light reaching the element from the end surface of the light guide member and configures the luminance distribution on the light emitting surface, the luminance distribution being obtained by reducing a light distribution pattern to be projected (and in some cases horizontally and vertically reversing the pattern).