Fast tailward ion flows with strongly southward magnetic fields are frequently observed near the neutral sheet in the premidnight sector of the magnetotail at 20--30 RE for substorm onsets in Geotail observations. These fast tailward flows are occasionally accompanied by a few keV electrons. With these events, we study the structure and dynamics of magnetic reconnection. The plasma sheet near the magnetic reconnection site can be divided into three regions: the neutral sheet region (near the neutral sheet with the absolute magnitude of Bx of 10 nT), and the off-equatorial plasma sheet (the rest). In the neutral sheet region, plasmas are transported with strong convection, and accelerated electrons show nearly isotropic distributions. In the off-equatorial plasma sheet, two ion components coexist: ions being accelerated and heated during convection toward the neutral sheet and ions flowing at a high speed almost along the magnetic field. In this region, highly accelerated electrons are observed. Although electron distributions are basically isotropic, high-energy (higher than 10 keV) electrons show streaming away from the reconnection site along the magnetic field line. In the boundary region, ions also show two components: ions with convection toward the neutral sheet and field-aligned ions flowing out of the reconnection region, although acceleration and heating during convection are weak. In the boundary region, high-energy (10 keV) electrons stream away, while medium-energy (3 keV) electrons stream into the reconnection site. Magnetic reconnection usually starts in the premidnight sector of the magnetotail between XGSM=-20 RE and XGSM=-30 RE prior to an onset signature identified with Pi 2 pulsation on the ground. Magnetic reconnection proceeds on a timescale of 10 min. After magnetic reconnection ends, adjacent plasmas are transported into the postreconnection site, and plasmas can become stationary even in the expansion phase. ¿ 1998 American Geophysical Union