There are provided a method and apparatus for determining a geomagnetic field by using a compass and a method and apparatus for determining an azimuth angle of a moving object using the method and apparatus for determining a geomagnetic field. The method of determining a geomagnetic field by using a compass includes defining an allowable magnetic field range used to determine a valid geomagnetic region; calculating a magnitude of a magnetic field in a geomagnetic-field validity test region by using a compass; and if the magnitude of the magnetic field is within the allowable magnetic field range, determining that the geomagnetic-field validity test region is the valid geomagnetic region, and if not, determining that there is applied an external magnetic field disturbance. Accordingly, since an error of an azimuth angle of the compass due to an external magnetic field disturbance can be determined, it is possible to accurately detect a valid azimuth angle of the compass. In addition, since an allowable magnetic field range is determined based on a variation of magnetic field detected by a compass rather than a conventional method that simply compares a magnetic field and a geomagnetic field, it is possible to accurately determine the geomagnetic field.