A map database stores three-dimensional polygons of features, as well as water system polygons such as sea and lake and ground surface polygons. The map database stores map data in multiple levels having different levels of details, such as levels LVa to LVc. A procedure of displaying a three-dimensional map offsets the water systems relative to the ground surfaces, draws a map of a distant view area distant away from the viewpoint using map data at a low level of details, subsequently clears a depth buffer and newly draws a map of a close view area close to the viewpoint using map data at a high level of details. The offset is set to increase in the distant view area and decrease in the close view area. Increasing the offset in the distant view area avoids the occurrence of Z-fighting in the distant area from the viewpoint.