There is provided a single particle detection technique based on a scanning molecule counting method, enabling individual detection of a single particle using light measurement with a confocal or multiphoton microscope, and quantitative observation of conditions or characteristics of the particle. The inventive technique of detecting a single particle in a sample solution detects light containing substantially constant background light from a light detection region with moving the position of the light detection region of the microscope in a sample solution to generate time series light intensity data; and detects individually a light intensity reduction occurred when a single particle which does not emit light (or a particle whose emitting light intensity in a detected wavelength band is lower than the background light) enters in the light detection region in the time series light intensity data as a signal indicating the existence of each single particle.