Passive Image Interferometry (PII) is a seismological method employing ambient seismic noise to monitor the temporal evolution of mean shear wave velocity within the Earth. First, the elastic Green's tensor between two seismometers is computed from the cross correlation of seismic noise recorded during a certain period. Second, the constructed seismograms of different time periods are treated as earthquake multiplets, and small time shifts in their coda are used to invert a relative change in mean shear wave velocity. When applied to the source region of the 2004 M _w = 6.6 mid-Niigata earthquake, Japan (centroid depth 5 km), we used noise recorded at six seismometers located at a distance of less than 25 km from the epicenter. Daily inversions during the 2 months before and after the earthquake show a sudden drop of mean shear wave velocity of some tenths of a percent at the time of the mid-Niigata earthquake. Using noise in two frequency bands, 0.1–0.5 Hz and 2–8 Hz, we find similar amplitudes for the velocity drop, which indicates that changes are not restricted to the shallow subsurface. Possible interpretations of this velocity decrease are a decrease of crustal stress after the earthquake, a nonlinear site response in the shallow subsurface layer due to strong ground motion, or structural weakening due to the creation of new fractures in the source area of the earthquake.