An optical analysis device for detecting light from a light-emitting particle dispersed and moving at random in a sample solution, comprising: an optical system of a confocal microscope or a multiphoton microscope; light detection region moving means (17, 17a) for moving a position of a light detection region of the optical system in a flat plane in the sample solution at a speed greater than the speed of particle movement due to Brownian motion; light detecting means for detecting light from the light detection region; and signal processing means for generating time series light intensity data of the light from the light detection region detected with the light detecting means with moving the position of the light detection region in the sample solution, for detecting a signal indicating light from each light-emitting particle individually in the time series light intensity data, and for controlling the light detection region moving means (17, 17a) to sequentially move the position of the light detection region along at least a second route, to displace the light detection region along at least a first route and to move the position of the light detection region along at least one further second route identical to the previous at least one second route but displaced from it by the displacement along the at least one first route; characterized in that the signal processing means is capable of determining that only one light emitting particle has passed through the light detection region based on the signal having a bell shaped profile of light intensity versus time such that the time width for which the light intensity value continuously exceeds an appropriately set threshold value lies within a predetermined range, the first and the second routes are cyclic routes, and a moving cycle time of the position of the light detection region along the second route is shorter than a moving cycle time of the position of the second route along the first route. an optical system of a confocal microscope or a multiphoton microscope; light detection region moving means (17, 17a) for moving a position of a light detection region of the optical system in a flat plane in the sample solution at a speed greater than the speed of particle movement due to Brownian motion; light detecting means for detecting light from the light detection region; and signal processing means for generating time series light intensity data of the light from the light detection region detected with the light detecting means with moving the position of the light detection region in the sample solution, for detecting a signal indicating light from each light-emitting particle individually in the time series light intensity data, and for controlling the light detection region moving means (17, 17a) to sequentially move the position of the light detection region along at least a second route, to displace the light detection region along at least a first route and to move the position of the light detection region along at least one further second route identical to the previous at least one second route but displaced from it by the displacement along the at least one first route; characterized in that the signal processing means is capable of determining that only one light emitting particle has passed through the light detection region based on the signal having a bell shaped profile of light intensity versus time such that the time width for which the light intensity value continuously exceeds an appropriately set threshold value lies within a predetermined range, the first and the second routes are cyclic routes, and a moving cycle time of the position of the light detection region along the second route is shorter than a moving cycle time of the position of the second route along the first route.