An excellent example of an onset of the right-handed ion/ion resonant instability is observed on October 8, 1993, when the Geotail was in the distant tail of XGSM=-142 RE. Preceding and following a passage of a plasmoid, magnetic noises having a right-hand circular polarity with the frequencies around the proton cyclotron frequency are detected. Especially, the event after the passage lasted for more than 10 min., and the ion plasma in this boundary layer was composed of a tailwards streaming cold dense plasma and a warm tailward beam coming presumably from the plasma sheet, drifting at 1000 km/s relative to the former along the field line. We have used this distribution as a source function for a linear Vlasov analysis, and confirmed that the generation mechanism of the waves can well be explained in terms of the resonant interaction with the beam. ¿ American Geophysical Union 1994