When an image signal suitable for performing automatic adjustment of an effective image area and a dot clock is not output, the adjustment may be unsuccessfully performed. An effective image area detector detects an effective image area and determines whether an image in the effective image area is a blank image. If a synchronization signal detector determines that there exists a synchronization signal and if the effective image area detector determines that the image is not a blank image, a controller adjusts an area captured as image data by an input signal processor.