A gradient calculation unit calculates an image gradient on a pixel-by-pixel basis in each of a plurality of images. A scale search unit calculates a scale score based on degree of spatial frequency components for each pixel in each of the images. A gradient score calculation unit calculates, for each pixel, a gradient score on the basis of the image gradient and the scale score. A gradient finalization unit calculates, using the gradient score and, as a target gradient, an image gradient of each pixel in a combined image. On the basis of the target gradient, a coefficient calculation unit calculates, for each pixel of the combined image, a coefficient of base vectors constituting a preset base vector group in the combined image. A combining unit generates the combined image through linear combination of the base vectors based on the calculated coefficients.