PURPOSE: To obtain an efficiently usable particle detector by transforming a laser beam emitted from a laser light oscillating section into a sheet-like beam through a simple optical system and preventing the quantity of the laser beam from becoming nonuniform in the directly advancing direction of the laser beam, and then, by reducing the loss of the laser light emitted from the laser beam oscillating section.

CONSTITUTION: This particle detector is constituted of an optical system composed of a laser beam oscillating section 1, collimator lens group 2 which collimates the laser beam from the section 1, anamorphic prism 3a which transforms the laser beam passed through the lens group 2 to a sheet-like beam, beam stopper 6 which receives the sheet-like laser beam, and scatter light detector 5 which detect scattered light produced by particles which cross the sheet- like laser beam when dropping.