The Kashiwazaki-Kariwa nuclear power plant suffered extreme shaking during the 2007 Niigata-ken Chuetsu-oki earthquake. Accelerograms observed at the dense seismometer array in the plant are now open to the public and will provide valuable knowledge. The temporal changes of S-wave velocity were examined by using normalized input–output minimization (NIOM) method based on the vertical array records observed during the mainshock and the events before and after it. It was found that the S-wave velocity in the layers (surface–50 m depth and 50–100 m depth) decreased significantly during the principal motion of the mainshock (indicating nonlinear behavior), whereas nearly linear behavior was observed in the bedrock layer (below 100 m depth). It was also found that the S-wave velocity increased in the layers above 100 m depth soon after this motion, indicating no major liquefaction in these layers. Finally, the relationship between the shear modulus and shear strain was examined based on the obtained S-wave velocities. The normalized shear moduli (G/G0) in the surface and intermediate layers decreased to about 0.2 at a strain level of 1×10-3 to 1×10-2 and about 0.6 at a strain level of 1×10-3 to 2×10-3.