Extremely intense degassing at Miyakejima volcano (Japan) was observed after the 2000 eruption. The H2O emission rate was estimated, using a record of infrared thermal images, at 400 kton/day in November 2000. By April 2002, H2O emission rate had decreased exponentially to 100 kton/day. The H2O emission rate varied in parallel with the SO2 emission rate except for the early stage in September 2000. The ratio of H2O to SO2 emission rate after November 2000 remained constant at about 10, which is consistent with the H2O/SO2 ratio in the magma. The large rate of H2O and SO2 emission requires efficient degassing at shallow depth. The agreement between the gas composition of the plume and that of dissolved volatiles in melt inclusions indicates H2O discharged from the vent was mostly magmatic in origin after November 2000.