A particle size distribution measuring device that irradiates with light a particle group which is a measurement target and which is accommodated in a cell suitable for being disposed in the particle size distribution measuring device and that calculates a particle size distribution of the particle group on the basis of a light intensity signal indicating a light intensity of diffracted and/or scattered light resulting from the particle group, comprising: a light source configured to radiate the particle group in the cell with the light; a plurality of photodetectors configured to detect the light intensity of the diffracted and/or scattered light, and a computing device, wherein the computer device includes: an actual spectrum obtaining unit that obtains an actual spectrum which is a light intensity spectrum obtained from the light intensity signal; a non-target spectrum calculating unit that receives non-target particle size distribution data indicating a particle size distribution of a non-target particle group which is not the measurement target and which is accommodated in the cell and that calculates, on the basis of the non-target particle size distribution data, a non-target spectrum which is a light intensity spectrum to be obtained by irradiating the non-target particle group with light; a non-target spectrum removing unit that calculates a target spectrum which is a light intensity spectrum obtained by subtracting an influence of the non-target spectrum from the actual spectrum; and a target particle size distribution calculating unit that calculates the particle size distribution of the particle group which is the measurement target on the basis of the target spectrum, characterised in that the non-target spectrum calculating unit calculates the non-target spectrum by using the non-target particle size distribution data and a refractive index of the non-target particle group. a light source configured to radiate the particle group in the cell with the light; a plurality of photodetectors configured to detect the light intensity of the diffracted and/or scattered light, and a computing device, wherein the computer device includes: an actual spectrum obtaining unit that obtains an actual spectrum which is a light intensity spectrum obtained from the light intensity signal; a non-target spectrum calculating unit that receives non-target particle size distribution data indicating a particle size distribution of a non-target particle group which is not the measurement target and which is accommodated in the cell and that calculates, on the basis of the non-target particle size distribution data, a non-target spectrum which is a light intensity spectrum to be obtained by irradiating the non-target particle group with light; a non-target spectrum removing unit that calculates a target spectrum which is a light intensity spectrum obtained by subtracting an influence of the non-target spectrum from the actual spectrum; and a target particle size distribution calculating unit that calculates the particle size distribution of the particle group which is the measurement target on the basis of the target spectrum, characterised in that the non-target spectrum calculating unit calculates the non-target spectrum by using the non-target particle size distribution data and a refractive index of the non-target particle group. an actual spectrum obtaining unit that obtains an actual spectrum which is a light intensity spectrum obtained from the light intensity signal; a non-target spectrum calculating unit that receives non-target particle size distribution data indicating a particle size distribution of a non-target particle group which is not the measurement target and which is accommodated in the cell and that calculates, on the basis of the non-target particle size distribution data, a non-target spectrum which is a light intensity spectrum to be obtained by irradiating the non-target particle group with light; a non-target spectrum removing unit that calculates a target spectrum which is a light intensity spectrum obtained by subtracting an influence of the non-target spectrum from the actual spectrum; and a target particle size distribution calculating unit that calculates the particle size distribution of the particle group which is the measurement target on the basis of the target spectrum, characterised in that the non-target spectrum calculating unit calculates the non-target spectrum by using the non-target particle size distribution data and a refractive index of the non-target particle group.