To obtain a meaningful anomaly map showing magnetic fields of crustal origin from satellite observations, it is necessary to account adequately for and subtract the fields generated by the currents flowing in the ionosphere. Conventionally, corrections are made separately for each path by empirical methods of fitting and subtracting linear, quadratic, or harmonic functions from the original data. This procedure, however, lacks a sound physical basis. We examined the field components after the main field and the magnetospheric field were subtracted from Magsat data and found certain structures in the residual field. The residual fields at 0600 and 1800 local times were separately averaged over longitude to obtain the effective contribution of ionospheric current system at these local times, which is called the mean ionospheric field (MIF). To obtain the crustal anomalies, the MIF was subtracted from the residual fields. To account for the day-to-day variation in the ionospheric current system and for the difference in satellite altitude, anomaly maps are constructed on 5¿¿5¿ grid averages. The dawn and dusk anomaly maps after the correction agree well with each other, showing that the effect of the ionospheric field is well taken care of by the MIF correction. The amplitude of anomalies is large even at comparatively long wavelength, and the maps successfully represent these crustal anomalies which have been greatly reduced in the previous maps.