Light source device and vehicle lighting device

An LED light source device can have a simple configuration which can easily create a desired light distribution pattern with a low profile and light weight. An illumination apparatus and a vehicle lighting device can also be configured to use the LED light source device. The light source device can include a light guide plate made of a flat plate-like material that is transparent in the visible range, and which has a front surface serving as a light emission surface. A point light source can be opposed to an end surface of the light guide plate. A rear surface of the light guide plate can include a luminance control element configured to control a luminance distribution on the light emission surface. The luminance control element controls light from the light source, incident through the end surface of the light guide plate, so that a reduced inversion of a light distribution pattern that is to be emitted is formed on the light emission surface as the luminance distribution. The light guide plate can include a polarizing film for making the light emission surface emit p-polarized (parallel polarized) light.