PURPOSE: To make it possible to measure the particle size distribution of fine particles, by performing measurement after particles to be measured were adsorbed by particles having a measurable particle size.

CONSTITUTION: When particles to be measured, for example, albumin particles A are dissolved in water B to be received in a container, because the specific gravity of albumin particles A is almost the same to that of water B, measurement is impossible even if the resulting solution is set to a centrifugal sedimentation type particle size distribution measuring apparatus. Therefore, an adsorbent, for example, a silica particles C having known particle size distribution and specific gravity higher than that of the solution are mixed with the solution under stirring and the resulting mixture is set to the measuring apparatus at the point of time when the albumin particles A were adsorbed by silica particles C and revolved. Because the silica particles C' adsorbing the albumin particles A become larger in the particle size thereof and lower in the specific gravity thereof as compared with those before adsorption, the silica particles C' are lowered in the sedimentation speed as compared with the silica particles C alone. Because the change quantity of this sedimentation speed relates to the particle size of albumin particles A, by measuring said change quantity, the particle size of albumin itself can be calculated.