The particle size distribution measurement device irradiates light onto a sample of particles, then detects secondary light generated by this irradiation, and then calculates a particle size distribution of the particles based on the detection data, and includes a separate measurement data receiving unit that receives separate measurement data obtained by separately measuring particles having a specific particle size in a separate sample, and light intensity data showing a light intensity of the secondary light generated by the particles in the separate sample, and a distribution conversion unit that, based on the separate measurement data and on the light intensity data, converts the particle size distribution from a distribution in which the numbers of particles of each particle size contained in the sample being measured are shown in relative terms to a distribution in which the numbers of these particles are shown in absolute terms.