Along with the progress of processing technology on substrate, devices having a finer structure are being manufactured. Particularly in semiconductor manufacturing sites, the minimum unit has reached about 10 nm, and the existence of particulate defects of several nm affects the yield, so inspection techniques with higher sensitivity than conventional methods are required. Therefore, we propose an optical inspection method of nanoparticle defect based on a new principle that liquid is used as a probe. In this report, we introduce the phase contrast microscope for high sensitivity of the proposed method and report on the result of characteristic analysis.