To provide a particle size distribution measuring instrument capable of accurately measuring a sample with high reproducibility, even if the amount of sample is small, and to provide a particle size distribution measuring method.

The particle size distribution measuring instrument is equipped with a sample supply mechanism 2; a sample recovery mechanism 3; a diffracted scattered light intensity distribution data repeated measuring part 41 which measures the space intensity distribution data of diffracted/scattered light, stops measurement if the sample supplied from the sample supply mechanism is exhausted, and which resumes measurement of the space intensity distribution data of the diffracted/scattered light to repeat the measurement of the space intensity distribution data of the diffracted/scattered light and the stoppage the measurement by a plurality number of times; a diffracted/scattered light intensity distribution integrating part 42 for integrating the space intensity distribution data of the diffracted/scattered light repeatedly measured by a plurality number of times; and a particle size distribution calculating part 43 for calculating the particle size distribution, based on the integrated diffracted/scattered light intensity distribution data.