A method for measuring particulate sizes of nanometer (nm) order on an ultraprecisely-machined surface has been developed using a laser light scattered method. In this paper, we proposed a new method to detect particles of a 6 nm particulate diameter on a raw Si wafer, and developed the measuring system having a detection sensitivity of a 19 nm particulate size with this measuring theory. This system is more than one order of magnitude higher than that of the conventional method. This, a new system was developed with a CCD sensor to simplify the setting of the optical system and with automated measuring apparatus for application in the ultra-clean room of class 1. To evaluate this measuring system, the measurement of particulate sizes was attempted with a standard particle. It was verified that this measuring system could measure the particulate size in a detection sensitivity of 24nm on an ultraprecisely-machined surface. Furthermore, it was used to detect particles on the surface of the raw Si wafer ; particles corresponding to a particulate diameter about of approximately 24-34 nm could be detected. In addition, to verify the measured particle, wet cleaning for the Si wafer was attempted in the ultra clean room. Consequently, it was verified that detected particles cn the raw Si wafer were foreign particles and that the wet cleaning is an effective method to remove particles of less than 0.1μm diameter on Si wafer surface.