Abstract: An apparatus for measuring the size distribution of particles by making use of centrifugal force field, comprising a mechanism for making a suspension revolve, an optical unit for detecting a light absorption by the suspension, and a micro-computer. The suspension suspends test particles whose size distribution is to be measured. The optical unit measures a time-dependent particle concentration variation due to the particle movements forced in a centrifugal force field produced by the revolution of the suspension. The micro-computer is provided with a memory for storing equations to obtain the size distribution of particles from the measured particle concentration variation and equations for correcting adverse effects by non-parallel particle movements in a centrifugal force field, to give a particle size distribution free from the error arising from the non-parallel movements of particles.