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