A game apparatus generates an image of a three-dimensional virtual space as viewed from a predetermined viewpoint, an object being provided in the three-dimensional virtual space. Next, the game apparatus obtains a depth value (Z value) indicating a depth from the viewpoint in a line-of-sight direction in the three-dimensional virtual space for each pixel of the image. Further, the game apparatus detects a pixel corresponding to a contour of the object based on the depth value. Specifically, a plurality of pixels are specified as reference pixels. It is determined whether or not a pixel of interest corresponds to a contour, based on a change level of differences in depth value between the pixel of interest and the reference pixels.