A particle monitoring apparatus includes a housing disposed on a gas exhaust line, a laser beam source for emitting a laser beam to particles in the gas exhaust line, a window member disposed at the housing for monitoring the particles in the gas exhaust line. The window member has a transparent base which is formed of a transparent resin or glass containing silicon and has a gas contact surface which faces a gas within the gas exhaust line, and a surface treatment layer formed on the gas contact surface of the transparent base, wherein the surface treatment layer contains one material selected from the group consisting of yttrium and calcium fluoride.