Mount Sakurajima, situated in the southern Kyusyu district of southwest Japan, is known as one of the most active volcanoes in Japan. It has erupted intermittently, ejecting large volumes of volcanic materials including lava, scoria, and ash. We have adopted a SAR interferometry method using JERS-1 images to monitor volcanic activity and estimate accompanying topographic changes in order to mitigate hazard from the inevitable eruptions and debris flows. A map expressing the phase-difference distribution for pairs of images recorded on two acquisition dates is produced for this purpose. This map, obtained by subtracting orbital and topographic fringes from the initial interferogram, is capable of quantifying vertical displacements at each point on the SAR image. Considering the acquisition-date intervals for the images used, three phase-difference maps are generated for the period from January 1996 to March 1997. These show that relatively large displacements occurring along the valleys on the northern slopes of the volcano are common to the three pairs. Because the volcanic activity was low during the period, the displacements may be caused chiefly by the erosion of the surface materials. Differential SAR interferometry is thus thought to be capable of estimating temporal fluctuations of topography during arbitrary periods, detecting regions that are readily aggraded or eroded, and helping to determine the position and dimensions of debris barriers.