In an image pickup apparatus 1, reference image data is generated from plural pieces of image data and further, at least two pieces of arbitrary image data are selected from among the plural pieces of image data. A first difference calculating unit 71 calculates differences between the selected pieces of image data for two adjacent pixels. A second difference calculating unit 72 calculates differences in pixel values of the selected pieces of image data from the reference image data. A cost function is set, having variables of the differences calculated by the first difference calculating unit and the differences calculated by the second difference calculating unit. A boundary setting unit 74 sets boundaries respectively in the selected pieces of image data so as to minimize the cost function. The selected pieces of image data are combined together in accordance with the boundaries set by the boundary setting unit.