When synthesizing a plurality of images that partially overlap one another to derive a larger image, the target larger image can be derived with less processing. First, a plurality of first images mutually including portions recording the same given subject are prepared (S2). Next, each first image is subjected to resolution conversion, to generate a second image with lower pixel density (S4). Then, based on portions recording the same subject, relative positions of the second images are calculated (S6). After that, an image generation area is determined, within a composite area which is the sum of areas recorded by the second images (S8). Then, first partial images, which are portions of the second images included in the image generation area, are determined (S10). After that, second partial images, which are part of the first images and correspond to the first partial images, are determined (S12). Finally, third images are generated based on the second partial images (S14).