1. An image processor for producing and displaying a display image showing a picture obtained by viewing a virtual three-dimensional space where a viewpoint and a plurality of display target objects are disposed, from the view point, comprising: means for calculating a display size, in the display image, of each of the plurality of display target objects; means for selecting some of the plurality of display target objects based on the calculated display size of each of the plurality of display target objects; and means for updating a state of the selected objects in the virtual three-dimensional space at a time interval different from that for objects other than said selected some of the plurality of display target objects in the virtual three-dimensional space, based on said calculated display size, wherein each of the plurality of display target objects is displayed in said display image.