When a moving body (e.g., a car) is entering a roundabout, a navigation machine extracts link data of the roundabout from road map data. The navigation machine detects a route entering the roundabout (i.e., a route the moving body is currently taking), a center of the roundabout, and an exiting route based on the link data to determine an angle of the exiting route with respect to the entering route. A direction mark having the exiting route angle is prepared as a heading direction guidance image. The navigation machine can show an appropriate route (direction) at the roundabout without increasing the amount of stored image data.