A computer readable storage device having a computer program product including programmed instructions for optical analysis of detecting light from a light-emitting particle dispersed and moving at random in a sample solution using an optical system of a confocal microscope or a multiphoton microscope, said programmed instructions causing a computer to perform steps comprising:
- moving a position of a light detection region of the optical system in the sample solution periodically along a predetermined route;
- measuring a light intensity from the light detection region during the moving of the position of the light detection region in the sample solution to generate light intensity data;
- detecting individually a signal indicating light from a single light-emitting particle in the light intensity data;
- and
- determining an index value indicating a translational diffusional characteristic of one light-emitting particle in a plane perpendicular to a moving direction of the light detection region based upon an intensity value of a detected signal indicating light of the one light-emitting particle and an intensity value within a time region separated from a generation time of the signal indicating light of the one light-emitting particle by a time corresponding to an integral multiple of a moving cycle of the position of the light detection region in the time series light intensity data.