A mosaic image which encourages users' motivation of continuously browsing mosaic images is generated. An image generation device which generates a mosaic image using as a source image a material image divided into a plurality of divided areas, has: an acquiring means that acquires an image from a terminal device through a network; and a mosaic image generating means that generates a mosaic image by allocating the image acquired by the acquiring means, to one of the divided areas based on color information of the image acquired by the acquiring means and color information of each of the divided areas of the material image used as the source image. After generating the mosaic image, the mosaic image generating means generates a new mosaic image by changing at least one of a number of the divided areas and the material image in response to an increase in a number of images acquired by the acquiring means.