An optical analysis device configured to detect light from a light-emitting particle dispersed and moving at random in a sample solution, wherein the device comprises: a confocal microscope or a multiphoton microscope, wherein the optical analysis device is configured to detect the light by using an optical system of the confocal microscope or the multiphoton microscope; a light detection region moving portion (7, 17) configured to move a position of a light detection region of the optical system in the sample solution; a light detecting portion configured to detect light from the light detection region; and a signal processing portion configured to generate time series light intensity data of the light from the light detection region detected with the light detecting portion during moving the position of the light detection region in the sample solution, and to detect individually a light signal from each light-emitting particle in the time series light intensity data; wherein the signal processing portion is configured to compute a characteristic value of light intensity indicating a presence or an absence of the light from a single light-emitting particle in every time section of a predetermined width in the time series light intensity data, and to judge individually if the light signal from the light-emitting particle exists in the time section of the predetermined width, using the characteristic value, characterized in that the light detection region moving portion (7, 17) is configured to move the position of the light detection region at a velocity quicker than a diffusional moving velocity of the light-emitting particle, and the predetermined width is set to be substantially longer than a time width required for an interval from a time of one light-emitting particle's entering into the light detection region to a time of its exiting therefrom, and also substantially shorter than a time width from a time of one light-emitting particle's entering into the light detection region to a time of a different light-emitting particle's entering into the light detection region. a confocal microscope or a multiphoton microscope, wherein the optical analysis device is configured to detect the light by using an optical system of the confocal microscope or the multiphoton microscope; a light detection region moving portion (7, 17) configured to move a position of a light detection region of the optical system in the sample solution; a light detecting portion configured to detect light from the light detection region; and a signal processing portion configured to generate time series light intensity data of the light from the light detection region detected with the light detecting portion during moving the position of the light detection region in the sample solution, and to detect individually a light signal from each light-emitting particle in the time series light intensity data; wherein the signal processing portion is configured to compute a characteristic value of light intensity indicating a presence or an absence of the light from a single light-emitting particle in every time section of a predetermined width in the time series light intensity data, and to judge individually if the light signal from the light-emitting particle exists in the time section of the predetermined width, using the characteristic value, characterized in that the light detection region moving portion (7, 17) is configured to move the position of the light detection region at a velocity quicker than a diffusional moving velocity of the light-emitting particle, and the predetermined width is set to be substantially longer than a time width required for an interval from a time of one light-emitting particle's entering into the light detection region to a time of its exiting therefrom, and also substantially shorter than a time width from a time of one light-emitting particle's entering into the light detection region to a time of a different light-emitting particle's entering into the light detection region.