There is provided an apparatus for monitoring a size of a particle, including (a) a laser beam source which radiates a laser beam to an area in which particles exist, (b) a photodetector which receives the laser beam having been scattered by the particles, and outputs image data including brightness of pixels, (c) an area detector which detects pixels corresponding to an area on which the scattered laser beam is incident, based on the image data, (d) a maximum brightness detector which detects a maximum brightness among brightness of the pixels detected by the area detector, and (e) a measurement unit which compares the maximum brightness to a predetermined threshold brightness to thereby measure a relative size of the particles.