In order to reduce a calculation time of a particle size distribution, a particle size distribution measuring apparatus includes: a light source for irradiating light to particles to be measured; a plurality of photodetectors for detecting light intensities of diffracted/scattered lights caused by the irradiation of the light; and an operation part for receiving light intensity signals outputted from the respective photodetectors and calculating a particle size distribution of the particles based on the fact that a vector s is represented by a predetermined expression including a product of a vector q and a coefficient matrix K, and the operation part is configured to compute in parallel and calculate at least two elements of the elements of the coefficient matrix K.