We have statistically studied the three-dimensional structure of evolution of the magnetotail associated with substorm expansion onsets. We selected 397 auroral breakups from Polar UVI data with a timing uncertainty of less than 2 min and then performed the two-dimensional superposed epoch analysis using GEOTAIL data with 2 min resolution. We also performed the regional superposed epoch analysis with 1 min resolution using only the events for which we could determine the onset time with an uncertainty of less than 1 min. In particular, we found the simultaneous variations in the north-south magnetic field, the duskward electric field, and the thermal flux immediately after onset around X ~ -10RE and X ~ -30RE, as well as the first total pressure decrease slightly (0--2 min) before onset around X ~ -20RE. These results suggest that the merging of magnetic field lines or the magnetic reconnection first occurs around X ~ -20RE in the premidnight tail at least 0--2 min before onset. Then, immediately after onset, the dipolarization occurs around X ~ -10RE, which is simultaneous with substantial evolution of the plasmoid around X ~ -30RE.