1. A three-dimensional image output device that outputs a three-dimensional image in which an object is drawn three-dimensionally with a background thereof, the three-dimensional image output device comprising: a three-dimensional model storage that stores a three-dimensional model representing a three-dimensional shape of the object, the three-dimensional model including map data representing three-dimensional shapes of ground surface and features; a projecting section that uses the three-dimensional model and generates a projection which is a three-dimensional object image that expresses the object three-dimensionally; a background layer generating section that generates a background layer representing the background located behind the object in a three-dimensional space, the background layer including a background image having a transparent portion and an opaque portion; and an image output controller that generates the three-dimensional image by superimposing the background layer in front of the three-dimensional object image contrary to an actual location of the background behind the object in the three-dimensional space, and outputs the three-dimensional image, the three-dimensional image being a three-dimensional map that expresses a geographical shape three-dimensionally, wherein at least one of a generating condition of the three-dimensional object image and a generating condition of the background layer is adjusted to cause the opaque portion of the background image to cover and hide part of the three-dimensional object image, wherein the background image has the opaque portion in an upper part of the background image, and the transparent portion in a lower part of the background image, wherein a boundary between the transparent portion and the opaque portion is expressed by a circular arc or an elliptical arc connecting a left side and a right side of the background image, the opaque portion showing the stratosphere of the earth, and wherein the projecting section generates the three-dimensional object image having the part covered and hidden by the opaque portion from a viewpoint position where a horizon is to be recognized as a curved line.