Temporal changes in the plasma flow and magnetic fields of the Earth's magnetotail in the region of -7.5>X(RE)>-52.5 and -15E)<15 in the GSM coordinates were studied by applying a superposed epoch analysis method to 263 substorm events observed by GEOTAIL spacecraft. The onset times of the substorms were determined with Pi-2 pulsations at Kakioka station. It was found that the tailward flow associated with plasmoids started about 2 or 3 min before the Pi-2 onsets, and the earthward flow became somewhat coherent 1 min before the onsets. The variation of the northward magnetic field component was investigated by obtaining the deviation of this component from that in the interval from 10.5 to 7.5 min before the Pi-2 onsets. We found that the northward magnetic field increased at X~-10 RE and decreased at X~-28 RE simultaneously in the premidnight tail immediately after the onsets. These variations definitely correspond to the dipolarization and plasmoid formation, respectively. ¿ 1999 American Geophysical Union