An image generating method for producing a pictorial image at higher speed when a three-dimensional object is arranged in an object space. The image generating method has: generating at least one of retouched image of a three-dimensional image by arranging a plurality of brush images so as to superpose a part of the plurality of brush images on one another part of the plurality of brush images within a rendering region on which the three-dimensional object is projected on the basis of a predetermined viewpoint; generating a projection image by projecting the three-dimensional object on the basis of the predetermined viewpoint; and rendering an image of the three-dimensional object so as to reflect color information of the projection image at a part at which the retouched image is transparent by synthesizing the retouched image with the projection image.