A car navigation system includes a distance sensor 2; an angular velocity sensor 1 for detecting angular velocity having a sensor detection axis in a vertical direction of a casing; an acceleration sensor 3 for detecting acceleration having a sensor detection axis in a back and forth direction of the casing on a horizontal plane; a body velocity and acceleration measuring unit 42 for measuring velocity and acceleration in the body direction of vehicle movement from the signal from the distance sensor; an angular velocity measuring unit 41 for measuring angular velocity from the signal from the angular velocity sensor; a body roll angle estimating unit 43 for estimating a body roll angle from the body direction of vehicle movement velocity and angular velocity; an acceleration sensor output estimating unit 45 for outputting, when the casing is set in the body with a rotation in a yaw direction, estimation values of the output signal of the acceleration sensor at every prescribed angle; and a yaw direction mounted angle detecting unit 46 for determining as a yaw direction mounted angle in the body an angle at a time when an estimated value is obtained which agrees most closely with a value remaining after subtracting from the signal from the acceleration sensor its offset component.