A correction apparatus for an azimuth calculation system which includes an azimuth sensor mounted on a moving body for producing two mutually perpendicular magnetic field components relative to the earth's magnetism acting thereon, and wherein the azimuth sensor is moved in a full circle so that an azimuth circle is formed by the two magnetic field components. The correction apparatus is arranged to store a plurality of interspaced X-axis lines in parallel with the abscissa of a right-angled coordinate and a plurality of interspaced Y-axis lines in parallel with the ordinate of the coordinate, to determine maximum and minimum values respectively defined by intersections of the azimuth circle with the X-axis lines and maximum and minimum values respectively defined by intersections of the azimuth circle with the Y-axis lines, to determine first and second center values of the azimuth circle based on each mean value of the first-named maximum and minimum values and the second-named maximum and minimum values, to correct the magnetic field components respectively in accordance with the first and second center values, and to calculate a direction of the moving body on a basis of the corrected magnetic field components.