In order to reduce a calculation time of a particle size distribution, a particle size distribution measuring apparatus includes an operation part for receiving light intensity signals outputted from 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 adapted to calculate values of a plurality of first parameters that depend on the particle sizes of the particles and a plurality of second parameters that depend on spread angles of the diffracted/scattered lights, wherein the first and second parameters are used for calculating one element among elements of the coefficient matrix K, and at least one of these calculated values is stored to be used for calculating another element of the coefficient matrix K.