A particle size measurement method, comprising: a test measurement step of irradiating a sample with light during a predetermined test measurement time period, wherein the test measurement time period is divided in a predetermined number of subsequent groups of measurement timings, wherein each group corresponds to a predetermined number of channels and a predetermined constant sampling time period of that group, wherein the predetermined constant sampling time period of the first group is chosen to be the smallest and the predetermined constant sampling time period of every subsequent group is chosen to be higher to thereby measure a test measurement intensity of scattered light scattered by the sample; an autocorrelation function calculation step of calculating an autocorrelation function representing a relationship between an autocorrelation of the test measurement intensity and time; a setting step of setting a smaller number of first to subsequent groups of measurement timings than the total number of groups corresponding to the test measurement time period to be used for main measurement, wherein the number of the last group as set to be used for main measurement is determined by calculating a time period required until the autocorrelation function falls below a predetermined threshold value and a preliminary time period set and added to the time period, wherein the last group is the group that includes that combined time period; a main measurement step of irradiating the sample with light during the first to last group as set in the setting step, defining the main measurement time period to thereby measure a main measurement intensity of the scattered light scattered by the sample; and a particle size calculation step of calculating a particle size of the sample based on the main measurement intensity. a test measurement step of irradiating a sample with light during a predetermined test measurement time period, wherein the test measurement time period is divided in a predetermined number of subsequent groups of measurement timings, wherein each group corresponds to a predetermined number of channels and a predetermined constant sampling time period of that group, wherein the predetermined constant sampling time period of the first group is chosen to be the smallest and the predetermined constant sampling time period of every subsequent group is chosen to be higher to thereby measure a test measurement intensity of scattered light scattered by the sample; wherein the test measurement time period is divided in a predetermined number of subsequent groups of measurement timings, wherein each group corresponds to a predetermined number of channels and a predetermined constant sampling time period of that group, wherein the predetermined constant sampling time period of the first group is chosen to be the smallest and the predetermined constant sampling time period of every subsequent group is chosen to be higher to thereby measure a test measurement intensity of scattered light scattered by the sample; an autocorrelation function calculation step of calculating an autocorrelation function representing a relationship between an autocorrelation of the test measurement intensity and time; a setting step of setting a smaller number of first to subsequent groups of measurement timings than the total number of groups corresponding to the test measurement time period to be used for main measurement, wherein the number of the last group as set to be used for main measurement is determined by calculating a time period required until the autocorrelation function falls below a predetermined threshold value and a preliminary time period set and added to the time period, wherein the last group is the group that includes that combined time period; wherein the number of the last group as set to be used for main measurement is determined by calculating a time period required until the autocorrelation function falls below a predetermined threshold value and a preliminary time period set and added to the time period, wherein the last group is the group that includes that combined time period; a main measurement step of irradiating the sample with light during the first to last group as set in the setting step, defining the main measurement time period to thereby measure a main measurement intensity of the scattered light scattered by the sample; and a particle size calculation step of calculating a particle size of the sample based on the main measurement intensity.