Calderas are ubiquitous topographic features of volcanos, yet caldera formation itself has not been recorded intensively by modern measurement techniques. Here we report the spatiotemporal gravity changes before and after caldera collapse at the Miyakejima volcano, Japan in 2000. A gravity decrease of as much as 145 ¿Gal (1 ¿Gal = 10-8 m/s2) at the summit area since June 1998 was detected 2 days prior to the collapse, interpreted as reflecting the formation of a large void beneath the volcano. Gravity changes detected after the initiation of collapse can mostly be corrected by the effect of collapsed topography, from which a rapid rate of collapse of more than 1.6 ¿ 107 m3/d can be inferred. Correcting for the effect of topography change, we identified a temporal decrease in gravity from the middle of July to late August despite ground subsidence. The gravity decrease is interpreted as a reduction of the density in a cylindrical conduit, attributed to water inflow from an ambient aquifer that also promoted intensive magma-water interaction and subsequent explosive eruptions. From September to at least November 2000, gravity values at all sites increased significantly by a degree that cannot be explained by ground displacement alone. We interpret this temporal evolution as primarily due to magma ascent and refilling of the conduit.