A magnetic inversion algorism for constructing a 3D-magnetic structure of a volcanic edifice was developed and applied to the airborne magnetic anomalies of Miyakejima Volcano surveyed in 1987, 1999, and 2001. The method consists of two steps. First, a uniformly magnetized terrain model is assumed for calculating the mean magnetization intensity, and the residual field was calculated by subtracting the terrain effect. In the second step, the magnetization deviations (ΔJ) from the mean value are calculated for each prismatic block constituting the three dimensional volcanic edifice, by the refined Conjugate Gradient iteration method under the condition of 10A/m＜ΔJ＜10A/m. Characteristics of the derived 3D-magnetic structures are summarized as foliowing four features. (1) Miyakejima Volcano has rather uniform magnetization, whose non-uniform magnetization is less than 10～20% of total bulk-magnetization (total summation of the product of block volumes and magnetization intensities).(2) A nearly N-S trending magnetic basement is estimated at the deeper part of the volcano.(3) Relatively weak magnetization zones are estimated beneath the eastern coastal zone and its offshore area, and beneath the western flank of the edifice.(4) A relatively high magnetization, zone is estimated beneath the northern offshore area. Comparison of the 3D-magnetic structures derived from surveys in 1987 and 1999 made apparent that the zone of relatively weak magnetization intensity had spread wider from 1987 to 1999, with the most demagnetized zone at the base (0m～300m b.s.1.) in about 750m southeast from the center of the crater. The 3D-magnetic structure in 2001 indicates the demagnetized zone is considerably restricted in comparison with that in 1999. This feature may suggest that the thermal demagnetization has been depressed after the eruption in 2000. These results suggest that the detection of demagnetization process associated with volcanic activity might be promising by conducting repeated airborne magnetic surveys.