Array measurements of microtremors were performed at six sites in the Sendai region. Using the microtremors we calculate phase velocities of Rayleigh wave by F-K spectral analysis in the frequency range from 0.5 to 3.0 Hz. Then we invert S-wave velocity structures from the seismological bedrock to the engineering bedrock by fixing the surface ground structures which are already known. As a result, well-controlled thicknesses of two layers are obtained at four sites. The thicknesses at the other two sites are estimated by using the microtremors together with the site responses derived from strong motion records.