Based on an observer's position and observing direction measured in the real world and positions of a virtual light source and virtual object in a virtual world, a brightness adjustment value of the virtual light source is adjusted in a way that, when light from the virtual light source is blocked by an observer, brightness of the virtual object observed by the observer is reduced as if the observer were observing in the real world. As a result, a mixed reality presentation system that achieves a sense of reality is provided.