There is provided an optical analysis technique enabling the detection of the condition or characteristic of a particle to be observed contained at a low concentration or number density in a sample solution. The inventive optical analysis technique uses an optical system capable of detecting light from a micro region in a solution, such as an optical system of a confocal microscope or a multiphoton microscope, to detect the light from the light-emitting particle to be observed while moving the position of the micro region in the sample solution (while scanning the inside of the sample solution with the micro region), thereby detecting individually the light-emitting particle crossing the inside of the micro region to enable the counting of the light-emitting particle(s) or the acquisition of the information on the concentration or number density of the light-emitting particle.