To certainly check operation in the so-called diffraction/scattering particle size distribution measuring instrument and to enhance action reliability by an extremely simple constitution.

This measuring instrument of a particle size distribution is equipped with a light source 3 for irradiating a particle group with light L, a plurality of scattered light detectors 4 for detecting the intensity distribution of the scattered light LS emitted from the particle group irradiated with the light L and a data processor 5 for calculating the particle size distribution of the particle group on the basis of the values of the scattered light intensity signals outputted from the respective scattered light detectors 4. The data processor 5 is constituted so as to respectively compare the value of the normal scattered light intensity signal having to be obtained from a standard sample with a predetermined concentration with the values of the actual scattered light intensity signals outputted from the respective scattered light detectors 4 by irradiating the standard sample with the light L and judges whether the respective scattered light signals are normal to output a judge result.

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