Method and apparatus for inspecting foreign substance

An inspecting method for inspecting particles, which comprises:

• projecting beams from an illumination system, arranged above a light-permeable body to be inspected with an angle of incidence of a first elevation angle, onto an upper face of the body;
• receiving a reflecting light and a scattering light generated by the beams with a photodetecting angle of a second elevation angle by using a photodetecting system;
• calculating a result of an expression w<2* sin .alpha.*t{tan[sin.sup.-1 {sin(90.degree.-.alpha.)/n}]-tan[sin.sup.-1 {sin(.theta.-90.degree.)/n}]}, from the angle of incidence .alpha. which is approximately 2.degree., the photodetecting angle .theta., a thickness t of the body, and a refractive index n of the body to a particle over the body, in order to obtain w;
• and
• setting a width of the beams of the illumination system, before the beams are projected, to be not larger than w calculated according to the aforementioned expression.