Disclosed is an optical analysis technique that can detect the state or properties of target particles in a sample solution that has a low concentration or number density of said target particles. Said optical analysis technique uses an optical system that can detect light from a microscopic region of a solution, such as a confocal microscope or a multiphoton microscope optical system. While changing the position of the microscopic region in the sample solution (i.e., while scanning the sample solution by microscopic regions), light from the target particles, which are light-emitting particles, is detected. This makes it possible to detect individual light-emitting particles traversing microscopic regions, making it possible to count the light-emitting particles and obtain information on the concentration or number density thereof.