To accurately correlate a measurement value corresponding to the rotating position of an eyeball with the position of a viewing point through a relatively small number of operations.

The apexes of a projecting polygon and a plurality of reference points either of which serves as one internal point are set on the screen of an image display 3, and a plurality of screen areas are set by a straight line starting from the internal reference point and passing through the other reference points. The rotating position of an eyeball when it gazes at each reference point is set as a standard point, and a plurality of eyeball areas are set by a straight line starting from the origin set when the eyeball gazes at the internal reference point, the straight line passing through the other standard point. After an area determining part 11 determines the eyeball area to which the detected rotating position of the eyeball belongs, a viewing-point position computing part 12 shows the rotating position of the eyeball using the composite vectors of a reference vector on the boundary line of that eyeball area and calculates components, and then determines the position of the viewing point by synthesizing the reference vector on the boundary line of the corresponding screen area from the components.