1. A projection image area detecting device comprising: a projection image area detecting unit configured to detect marker image areas in a photographed image that is obtained by photographing a projection image area including a projection image, the marker image areas including marker images that are used for identifying four corners of the projection image area; and a marker image generating unit configured to generate the marker images including a specific area where a color changes as time passes, wherein the projection image area detecting unit detects the marker image areas by identifying the color included in the marker image areas that continues to change as time passes, and wherein the marker image generating unit generates the marker images having simple patterns that are not periodic patterns, such that the marker images can be identified in the photographed image.