A new submicron particle counter using a low output He–Ne laser (unpolarized, 8 mW) is proposed forinsitumeasurement of particulates. The design is based on calculations and experimental checks of factors governing the signal‐to‐noise ratio. Right angle scattering is found to be the most suitalbe optical system for eliminating miscellaneous noise and fundamental noise components derived from the carrier gas. The present version of the instrument with nine channels is constructed so as to minimized the noise level by reducing the sensing volume. This instrument can actually measure correct size distributions from 0.1 to 10 &mgr;m without detectable coincidence loss, at environmental particulate concentrations of up to 5×104/cm3.