A particle size distribution measuring apparatus comprising: a flow cell configured such that a plurality of portions of a sample including a medium and particles dispersed in the medium can be successively introduced; a light source configured to emit measurement light; a photodetector configured to receive scattered light from particles irradiated with the measurement light and to detect an intensity of the scattered light; a filter member that is arranged in a predetermined position in a light path of the measurement light from the light source to the flow cell or a light path of the scattered light from the flow cell to the photodetector and configured to attenuate the measurement light or the scattered light; and a measurement part configured to measure, on a basis of a fluctuation in the scattered light intensity detected by the photodetector and an attenuation level of the filter member, a particle size distribution of a portion of the sample in the flow cell, wherein: the measurement part is configured to, by measuring a particle size distribution every time a portion of the sample is introduced into the flow cell as well as by combining particle size distributions obtained at respective times of measurement, calculate a particle size distribution of the sample based on the measured portions of the sample, the portions being introduced at the multiple times of measurement, respectively; and the filter member is configured to be changeable to one having a different attenuation level at every time of measurement. a flow cell configured such that a plurality of portions of a sample including a medium and particles dispersed in the medium can be successively introduced; a light source configured to emit measurement light; a photodetector configured to receive scattered light from particles irradiated with the measurement light and to detect an intensity of the scattered light; a filter member that is arranged in a predetermined position in a light path of the measurement light from the light source to the flow cell or a light path of the scattered light from the flow cell to the photodetector and configured to attenuate the measurement light or the scattered light; and a measurement part configured to measure, on a basis of a fluctuation in the scattered light intensity detected by the photodetector and an attenuation level of the filter member, a particle size distribution of a portion of the sample in the flow cell, wherein: the measurement part is configured to, by measuring a particle size distribution every time a portion of the sample is introduced into the flow cell as well as by combining particle size distributions obtained at respective times of measurement, calculate a particle size distribution of the sample based on the measured portions of the sample, the portions being introduced at the multiple times of measurement, respectively; and the filter member is configured to be changeable to one having a different attenuation level at every time of measurement. the measurement part is configured to, by measuring a particle size distribution every time a portion of the sample is introduced into the flow cell as well as by combining particle size distributions obtained at respective times of measurement, calculate a particle size distribution of the sample based on the measured portions of the sample, the portions being introduced at the multiple times of measurement, respectively; and the filter member is configured to be changeable to one having a different attenuation level at every time of measurement.