In the scanning molecule counting method using optical measurement with a confocal or multiphoton microscope, there is provided a technique of computing a light-emitting particle concentration which changes with time and detecting a concentration change velocity or a reaction velocity. The inventive optical analysis technique of detecting light of light-emitting particles in a sample solution generates time series light intensity data of light from a light detection region detected with moving the position of the light detection region of the microscope in the sample solution; measures successively an interval of generation times of signals of the light-emitting particles detected in the time series light intensity data; and determines the concentration or concentration change velocity of the light-emitting particles, using the successively measured signal generation time intervals.