An extracting unit extracts a first sub-image set that satisfies a second condition and a second sub-image set that satisfies a third condition from an image set that satisfies a first condition. A similarity calculating unit calculates a similarity between images in the first sub-image set and a similarity between images in the second sub-image set. A selecting unit selects, as a first output image set, a plurality of images including a first image included in the first sub-image set and a second image included in the second sub-image set and selects, as a second output image set, a plurality of images including a third image included in the first sub-image set, the third image whose similarity to the first image being smaller than a threshold, and a fourth image included in the third sub-image set, the fourth image whose similarity to the second image being smaller than the threshold.