A ground surface as an image acquiring object is acquired by a synthetic aperture radar mounted on an artificial satellite at a usual time before occurrence of a disaster and thereby radar image data are obtained. After the occurrence of the disaster, an image of the ground surface as the image acquiring object is acquired within days shorter than the number of orbit returning days and by comparing this radar image data with the radar image data acquired at the usual time so as to try to early grasp a damaged situation. At a recovering and rebuilding time after the occurrence of the disaster, images of the image acquiring object are periodically acquired so as to prepare a recovering and rebuilding plan and prevent secondary disasters.