We are building an 8192x10240 CCD mosaic camera for 8m Japanese Telescope (Subaru). The mosaic will consist of 2x5 arrays of 3-edge buttable 4096x2048 15 mu m pixel imagers. Although several vendors have just started supplying the type of large format CCD, it is still in the development phase. Therefore, careful characterization and optimizations of individual CCD are critical. We describe the system used to evaluate several kinds of the CCDs. In addition to the CCD characterization. we also present the mechanical design of the mosaic focal plane which is an another issue to realize the large mosaic.