A particle measuring device for measuring a particle size of a target particle in a dispersion medium, comprising: an image analyzer configured to: acquire multiple images obtained at a predetermined time interval; specify, in a calibration mode, a mean-square displacement of a bright point of a calibration particle based on a displacement, in terms of a pixel unit, of the bright point of the calibration particle in the multiple images; and specify, in a measuring mode, a mean-square displacement of a bright point of the target particle based on a displacement, in terms of the pixel unit, of the bright point of the target particle in the multiple images; and a particle size analyzer configured to derive, in an analysis mode, the particle size of the target particle from the mean-square displacement of the bright point of the target particle based on the mean-square displacement of the bright point of the calibration particle and a particle size of the calibration particle, wherein: the image analyzer is configured to: specify, in the calibration mode, the mean-square displacement of the bright point of the calibration particle based on the displacement, in terms of the pixel unit, of the bright point of the calibration particle in the multiple images to calculate, as a device constant, a product of the mean-square displacement of the bright point of the calibration particle and a particle size of the calibration particle; and specify, in the measuring mode, the mean-square displacement of the target particle based on the displacement, in terms of the pixel unit, of the bright point of the target particle in the multiple images; and the particle size analyzer is configured to derive, in the analysis mode, the particle size of the target particle by dividing the device constant by the mean-square displacement of the bright point of the target particle. an image analyzer configured to: acquire multiple images obtained at a predetermined time interval; specify, in a calibration mode, a mean-square displacement of a bright point of a calibration particle based on a displacement, in terms of a pixel unit, of the bright point of the calibration particle in the multiple images; and specify, in a measuring mode, a mean-square displacement of a bright point of the target particle based on a displacement, in terms of the pixel unit, of the bright point of the target particle in the multiple images; and acquire multiple images obtained at a predetermined time interval; specify, in a calibration mode, a mean-square displacement of a bright point of a calibration particle based on a displacement, in terms of a pixel unit, of the bright point of the calibration particle in the multiple images; and specify, in a measuring mode, a mean-square displacement of a bright point of the target particle based on a displacement, in terms of the pixel unit, of the bright point of the target particle in the multiple images; and a particle size analyzer configured to derive, in an analysis mode, the particle size of the target particle from the mean-square displacement of the bright point of the target particle based on the mean-square displacement of the bright point of the calibration particle and a particle size of the calibration particle, wherein: the image analyzer is configured to: specify, in the calibration mode, the mean-square displacement of the bright point of the calibration particle based on the displacement, in terms of the pixel unit, of the bright point of the calibration particle in the multiple images to calculate, as a device constant, a product of the mean-square displacement of the bright point of the calibration particle and a particle size of the calibration particle; and specify, in the measuring mode, the mean-square displacement of the target particle based on the displacement, in terms of the pixel unit, of the bright point of the target particle in the multiple images; and the particle size analyzer is configured to derive, in the analysis mode, the particle size of the target particle by dividing the device constant by the mean-square displacement of the bright point of the target particle. the image analyzer is configured to: specify, in the calibration mode, the mean-square displacement of the bright point of the calibration particle based on the displacement, in terms of the pixel unit, of the bright point of the calibration particle in the multiple images to calculate, as a device constant, a product of the mean-square displacement of the bright point of the calibration particle and a particle size of the calibration particle; and specify, in the measuring mode, the mean-square displacement of the target particle based on the displacement, in terms of the pixel unit, of the bright point of the target particle in the multiple images; and specify, in the calibration mode, the mean-square displacement of the bright point of the calibration particle based on the displacement, in terms of the pixel unit, of the bright point of the calibration particle in the multiple images to calculate, as a device constant, a product of the mean-square displacement of the bright point of the calibration particle and a particle size of the calibration particle; and specify, in the measuring mode, the mean-square displacement of the target particle based on the displacement, in terms of the pixel unit, of the bright point of the target particle in the multiple images; and the particle size analyzer is configured to derive, in the analysis mode, the particle size of the target particle by dividing the device constant by the mean-square displacement of the bright point of the target particle.