1. A particle size distribution device for measuring the size of particles by irradiating the particles in a carrier medium with radiation that can be diffracted and/or scattered by the particles and detected by a plurality of detectors, comprising: a radiation unit for applying radiation to a measurement cell; a concentration level adjusting unit for changing a relative amount of particles to an amount of carrier medium to be applied to the measurement cell; a storage device for storing the outputs of the detectors for each concentration level irradiated; a correction unit for providing a concentration correction constant; and a calculating unit for providing particle size distribution outputs from the outputs of the detectors as adjusted by the concentration correction constant.