1. A particle size distribution measuring apparatus comprising: 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, wherein 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, the first and second parameters being used for calculating one element among elements of the coefficient matrix K, and store at least one of the values to be used for calculating another element of the coefficient matrix K, here, the vector s is a vector representing a light intensity pattern at every angle of the diffracted/scattered lights obtained from values of the light intensity signals outputted from the respective photodetectors, the vector q is a vector representing the particle size distribution of the particles to be measured, and the coefficient matrix K is a matrix for converting the vector q to the vector s.