A size distribution measurement device for measuring a size distribution of particles in a liquid sample containing the particles, the device comprising: a light source adapted to emit light; a scanning unit adapted to scan a focal point position of the light along an optical axis direction of the light and in a plane orthogonal to the optical axis direction for each focal point position of the light along the optical axis direction so that the light is reflected from respectively different positions in the optical axis direction of the particle in a state where the particle moves in the optical axis direction in the sample; a detector (117, 121) adapted to detect intensities of the light reflected from the sample; and a signal processing unit adapted to acquire an observation image of the sample in the plane orthogonal to the optical axis direction for each focal point position of the light scanned by the scanning unit along the optical axis direction and to calculate a size of each individual particle by using a maximum light intensity of the intensities of the light along the optical axis direction for each focal point position of the light, wherein the scanning unit is adapted to scan the focal point position at a frame rate of (γ × D) / (Δz × Δd) or more so that the scanning speed is fast enough to follow Brownian motion speed of the particle, where a scanning interval of the light in the optical axis direction is defined as Δd, a resolution of the size distribution measurement device in the optical axis direction is defined as Δz, a diffusion coefficient of the particles is defined as D, the number of scans per second of the focal point position of the light in the optical axis direction is defined as a frame rate and γ is a constant; characterized in that the signal processing unit is further adapted to perform a switching step of switching between a method of calculating the sizes of the particles by using the maximum light intensity and a method of calculating the sizes of the particles by using the acquired observation image, when a value obtained by dividing the size of one of the particles as calculated by using the maximum light intensity by a spot diameter of the light is equal to or greater than a first switching threshold value, the signal processing unit is adapted to calculate the size of the particle again by using the observation image, and when the size of the particle calculated by using the observation image is equal to or less than a second switching threshold value, the signal processing unit is adapted to calculate the size of the particle again by using the maximum light intensity. a light source adapted to emit light; a scanning unit adapted to scan a focal point position of the light along an optical axis direction of the light and in a plane orthogonal to the optical axis direction for each focal point position of the light along the optical axis direction so that the light is reflected from respectively different positions in the optical axis direction of the particle in a state where the particle moves in the optical axis direction in the sample; a detector (117, 121) adapted to detect intensities of the light reflected from the sample; and a signal processing unit adapted to acquire an observation image of the sample in the plane orthogonal to the optical axis direction for each focal point position of the light scanned by the scanning unit along the optical axis direction and to calculate a size of each individual particle by using a maximum light intensity of the intensities of the light along the optical axis direction for each focal point position of the light, wherein the scanning unit is adapted to scan the focal point position at a frame rate of (γ × D) / (Δz × Δd) or more so that the scanning speed is fast enough to follow Brownian motion speed of the particle, where a scanning interval of the light in the optical axis direction is defined as Δd, a resolution of the size distribution measurement device in the optical axis direction is defined as Δz, a diffusion coefficient of the particles is defined as D, the number of scans per second of the focal point position of the light in the optical axis direction is defined as a frame rate and γ is a constant; characterized in that the signal processing unit is further adapted to perform a switching step of switching between a method of calculating the sizes of the particles by using the maximum light intensity and a method of calculating the sizes of the particles by using the acquired observation image, when a value obtained by dividing the size of one of the particles as calculated by using the maximum light intensity by a spot diameter of the light is equal to or greater than a first switching threshold value, the signal processing unit is adapted to calculate the size of the particle again by using the observation image, and when the size of the particle calculated by using the observation image is equal to or less than a second switching threshold value, the signal processing unit is adapted to calculate the size of the particle again by using the maximum light intensity.