A fluorescence intensity compensation method, comprising: receiving, with photodetectors having different input wavelength bands, fluorescence emitted from fluorochromes excited by irradiating light on microparticles multiply-labeled by a plurality of fluorochromes with overlapping fluorescence wavelengths; collecting detected values for the photodetectors; and obtaining a measurement spectrum, by approximating, from a linear sum of single-stain spectrums obtained from microparticles individually labeled with the fluorochromes; wherein approximation of said measurement spectrum by the linear sum of said single-stain spectrums is performed using a restricted least-square method; and, wherein the intensity of fluorescence emitted from each fluorochrome is calculated by obtaining a parameter x j (j = 1 through M) where an evaluation function shown in the following Expression satisfies the following Expression and also is the smallest value χ 2 = ∑ i = 1 N p i − ∑ j = 1 M s ij ⋅ x j σ i 2 x j ≥ U j where S ij represents the detected value of the i'th photodetector in the single-stain spectrum of the j'th fluorochrome, p i represents the detected value of the i'th photodetector in the measurement spectrum, σ i represents an inverse number of a weight as to the detected value of the i'th photodetector, and U j represents a lower limit value of the fluorescence intensity of each fluorochrome to be calculated, wherein said lower limit value for a given fluorochrome is based on detection values from photodetectors obtained by irradiating particles not marked by that fluorochrome, characterized in that an undyed average value v j of the j'th fluorochrome obtained from the average value of detection values from each of said photodetectors, obtained by irradiating light on microparticles not marked with the j'th fluorochrome, is used as said lower limit U j. receiving, with photodetectors having different input wavelength bands, fluorescence emitted from fluorochromes excited by irradiating light on microparticles multiply-labeled by a plurality of fluorochromes with overlapping fluorescence wavelengths; collecting detected values for the photodetectors; and obtaining a measurement spectrum, by approximating, from a linear sum of single-stain spectrums obtained from microparticles individually labeled with the fluorochromes; wherein approximation of said measurement spectrum by the linear sum of said single-stain spectrums is performed using a restricted least-square method; and, wherein the intensity of fluorescence emitted from each fluorochrome is calculated by obtaining a parameter x j (j = 1 through M) where an evaluation function shown in the following Expression satisfies the following Expression and also is the smallest value χ 2 = ∑ i = 1 N p i − ∑ j = 1 M s ij ⋅ x j σ i 2 x j ≥ U j where S ij represents the detected value of the i'th photodetector in the single-stain spectrum of the j'th fluorochrome, p i represents the detected value of the i'th photodetector in the measurement spectrum, σ i represents an inverse number of a weight as to the detected value of the i'th photodetector, and U j represents a lower limit value of the fluorescence intensity of each fluorochrome to be calculated, wherein said lower limit value for a given fluorochrome is based on detection values from photodetectors obtained by irradiating particles not marked by that fluorochrome, characterized in that an undyed average value v j of the j'th fluorochrome obtained from the average value of detection values from each of said photodetectors, obtained by irradiating light on microparticles not marked with the j'th fluorochrome, is used as said lower limit U j.