PURPOSE: To measure reliably the inclination angle in the horizontal direction and the vertical direction of a camera by a method wherein the inclination of the camera is computed on the basis of the relationship between the position in a screen coordinate system of an object and the three-dimensional coordinates of a world coordinate system.

CONSTITUTION: In this method, three-dimensional coordinates of a specific object in a world coordinate system are found in advance, and the object is taken into by a camera. the position in a screen coordinate system of the object is specified, and the inclination angle of the camera in a relationship with the three-dimensional coordinates in the world coordinate system is detected. A plane OXZ is the screen coordinate system, the object which has been taken into from a lens in the camera forms an image on the plane PXZ. For example, when the camera is inclined in the horizontal direction and the vertical direction and not inclined in the rotation direction of the central axis (lens axis) of the camera, the inclination angle Φ in the vertical direction of the camera is the difference between the Y" axis of a camera coordinate system and the Z' axis of a lens coordinate system, and the inclination angle θ in the horizontal direction is expressed as the difference between the Y-axis of a world coordinate system and the Y" axis of the camera coordinate system.