The attribution information of a plurality of pieces of image data photographed by a plurality of photographing apparatus is obtained. The image data is classified into a predetermined time period on the basis of a photographing date and time obtained from the attribution information. The number of the photographing apparatus photographing the image data classified every predetermined time period is discriminated, and the image data classified into the time period in which the number of the pieces of image data is larger than a threshold value is extracted. The extracted image data is discriminated from the other image data to be displayed. Thereby, even if a plurality of pieces of still image data and moving image data photographed by a plurality of digital cameras and video cameras exists, a candidate of the image data relatively significant among them can be extracted easily.