A method of determining the three-dimensional position of an object in a specific position with respect to a predetermined standard position includes storing position data representative of a distinctive part of the object as if the object is virtually located in a predetermined standard position, formularizing a relation between the specific and standard positions in terms of a standard three-dimensional co-ordinate system as an equation including a positional deviation as an unknown parameter, and determining a restrictive condition for the position of the distinctive part of the object located in the specific position. The equation is solved under a condition in which the position of the distinctive part satisfies the restrictive condition to find a positional deviation.