The City of Yokohama has established a high-density strong-motion seismograph network in 1997 to enable rapid evaluation of seismic hazard in case of disastrous earthquakes. It consists of 150 surface strong-motion seismographs and nine borehole strong-motion seismographs within the city area, which is approximately 20 km × 30 km wide. We use seismograms recorded by the borehole seismographs of the network from seven earthquakes that occurred around Yokohama. We calculate the S-wave spectral ratio between the surface seismograph and the borehole seismograph and compare it with a synthetic spectral ratio calculated from a velocity profile of the borehole. The agreement of observed and synthetic spectral ratios is excellent, which shows that we may use these borehole seismographs to test various techniques for estimation of the site amplification. We then apply the horizontal-to-vertical (H/V) spectral-ratio technique for seismograms recorded at the surface and compare them with these spectral ratios. The results show that the H/V ratio technique gives a good estimate of the surface amplification for frequency range between about 1 and 4 Hz, if the near-surface velocity structure is relatively simple and if the signal-to-noise ratio of seismograms recorded at the surface is adequate.