Shear wave attenuation in deep soil deposits down to 2.3km in depth was directly and systematically measured at one of the deep-borehole observatories which were constructed for geophysical observations in the Tokyo metropolitan area.Introducing the newly designed SH-wave generator for an S-wave source, the down-hole measurements at 12 different depths were conducted down to 2.3km at intervals of 0.1-0.3km. Attenuation factors determined by the spectral ratio method were linearly inverted to QS-values under an assumption of frequency-independent QS.bottom of deep soil deposits, respectively, and larger than 150 in the substratum, for the frequency range of 3.5-20Hz.The QS for about 20 of the upper-most soil layer is faily consistent with known values which were estimated in shallower wells at various places.The substratum QS-value of 150 or over is not inconsistent with the crustal QS.The present measurement in such deep soil deposits is a new contribution to seismological and earthquake engineering studies.A significant effect of the attenuation factors upon the seismic waves is present by computing the amplification characteristics of deep soil deposits. At the present experimental site, the attenuation factor has a rather drastic effect upon the S wave in the period range shorter than 3sec.