The present invention provides a semiconductor manufacturing method, a plasma processing method and a plasma processing apparatus for generating a plasma in a processing chamber and carrying out processing on material to be processed by using the plasma, comprising a floating-foreign-particle measuring apparatus including: a light radiating optical system for radiating a light having a desired wavelength and completing intensity modulation at a desired frequency to the processing chamber; a scattered-light detecting optical system for separating a component with the desired wavelength from scattered lights obtained from the processing chamber as a result of radiation of the light by the light radiating optical system, for optically receiving the component and for converting the component into a first signal; and a foreign-particle-signal extracting unit which separates a second signal representing foreign particle floating in the plasma or in an area in proximity to the plasma from a third signal obtained by emission of the plasma for detection of the second signal by extraction of a component with the desired frequency used for the intensity modulation from the first signal obtained from the scattered-light detecting optical system.