A single particle detection device comprising a confocal microscope or a multiphoton microscope, the single particle detection device being configured to detect a single particle dispersed and moving at random in a sample solution using an optical system of the confocal microscope or the multiphoton microscope, the single particle detection device further comprising: light detection region moving means (17, 17a) configured to move a position of a light detection region of the optical system in the sample solution; light detecting means configured to detect light emitted from the light detection region; and signal processing means configured to control the light detection region moving means (17, 17a) to move the position of the light detection region, generate time series light intensity data of the light detected by means of the light detecting means while moving the position of the light detection region in the sample solution, and detect a signal indicating an existence of each single particle individually in the generated time series light intensity data by identifying a reduction of light intensity from a detected substantially constant background light in the time series light intensity data as the signal of a particle entering into the light detection region or being encompassed by the light detection region, wherein, when a time width of the light intensity being reduced below a set threshold value in the time series light intensity data lies within a predetermined range, the signal is judged to correspond to one particle having passed through the light detection region, and, when the time width of the light intensity being reduced below the set threshold value in the time series light intensity data does not lie within the predetermined range, the signal is judged as noise or a signal of a contaminant. light detection region moving means (17, 17a) configured to move a position of a light detection region of the optical system in the sample solution; light detecting means configured to detect light emitted from the light detection region; and signal processing means configured to control the light detection region moving means (17, 17a) to move the position of the light detection region, generate time series light intensity data of the light detected by means of the light detecting means while moving the position of the light detection region in the sample solution, and detect a signal indicating an existence of each single particle individually in the generated time series light intensity data by identifying a reduction of light intensity from a detected substantially constant background light in the time series light intensity data as the signal of a particle entering into the light detection region or being encompassed by the light detection region, wherein, when a time width of the light intensity being reduced below a set threshold value in the time series light intensity data lies within a predetermined range, the signal is judged to correspond to one particle having passed through the light detection region, and, when the time width of the light intensity being reduced below the set threshold value in the time series light intensity data does not lie within the predetermined range, the signal is judged as noise or a signal of a contaminant. control the light detection region moving means (17, 17a) to move the position of the light detection region, generate time series light intensity data of the light detected by means of the light detecting means while moving the position of the light detection region in the sample solution, and detect a signal indicating an existence of each single particle individually in the generated time series light intensity data by identifying a reduction of light intensity from a detected substantially constant background light in the time series light intensity data as the signal of a particle entering into the light detection region or being encompassed by the light detection region, wherein, when a time width of the light intensity being reduced below a set threshold value in the time series light intensity data lies within a predetermined range, the signal is judged to correspond to one particle having passed through the light detection region, and, when the time width of the light intensity being reduced below the set threshold value in the time series light intensity data does not lie within the predetermined range, the signal is judged as noise or a signal of a contaminant.