PURPOSE: To continuously perform the preparation of a suspension, natural sedimentation measurement and centrifugal sedimentation measurement by the same sample cell, by mounting the sample cell recirculating the suspension on a rotary body.

CONSTITUTION: A sample cell 1 provided with an inflow port 1a and an outflow part 1b is fixed to a freely rotatable and stoppable rotary body 2. The suspension of the outside is recirculated by a recirculation device consisting an inflow pipe 4a, an outflow pipe 4b and a recirculation pipe 6 through the inflow and outflow ports 1a, 1b and a medium liquid added while the concn. of the suspension is detected by a concn. detection means consisting of a light source 11 and a photoreceptor 12 to perform the conditioning of the suspension. Natural sedimentation measurement calculating the particle size distribution of a sample particle is performed by a microcomputer 16 using the change of concn. with the elapse of time after the stoppage of the driving of the recirculation device. In this state, the rotary body 2 is rotated by a motor 3 to perform centrifugal sedimentation measurement. By this method, measurement becomes possible on-line and the cost of the apparatus can be reduced.