We examine in detail the rapid decrease in Bz just before dipolarizations observed by the Geotail satellite in the near-Earth plasma sheet at (XGSM, YGSM) = (-8.3 RE, -5.1 RE). The observations were made using high-time-resolution data from a fluxgate magnetometer (16-Hz sample), a search-coil magnetometer (128 Hz), and an electric field antenna (64 Hz). Two dipolarizations were observed during a short time interval of 2 min. The magnetic Bz component suddenly decreased 2--4 s prior to the dipolarization. Characteristic waves with frequencies of 5--20 Hz and amplitudes of 1--3 mV m-1 and 5--15 nT s-1 were observed in the electric and magnetic field data at the time of the sudden decreases in Bz. We discuss two possible causes of the sudden decreases in Bz prior to the dipolarizations: (1) passage of a field-aligned line current associated with the substorm current wedge and (2) explosive growth phase and subsequent disruption of the tail current caused by the observed characteristic field oscillations.