An analytical apparatus comprising: a light source; a flow cell to flow a sample receiving light from the light source and emitting light; a spectral device to diffract the light emitted from the sample; a detection section (680; 104) provided with a plurality of detectors to detect the diffracted light outputted from the spectral device at the respective different wavelength; a signal processing unit to process a signal of the light detected by the detection section; an input unit for inputting an instruction to the signal processing unit; a storage unit to store a result of the processed signal; an outputting unit to display the result; and a band-setting unit to set, via the input unit, wavelength bands to be detected by the detectors; a wavelength intensity calculation unit to calculate the intensities of the signal of the detected light in the wavelength bands set by the band-setting unit, and to store the intensities for the respective samples in the storage unit; a cutoff value setting unit to output the intensities of the signal of the detected light of the wavelength bands for the respective samples, which intensities are stored in the storage unit with the wavelength intensity calculation unit, to the outputting unit and to make the input unit set certain values of the intensities based on which a signal whose intensity is lower than or equal to these certain values is rejected and to store the set certain values of the intensity in the storage unit; wherein the storage unit stores the signals of the detected light from each of a plurality of different calibration samples in the respective detectors and the intensities of the signals of the detected light at the respective detectors from each of the respective calibration samples are displayed on the outputting unit so that a user can set wavelength bands to be measured; the apparatus further comprising an estimated measurement accuracy calculation unit adapted to calculate a reject rate corresponding to the values of the intensities set through the cutoff value setting unit, wherein the reject rate is the proportion of particles relative to the total number of particles in the calibration samples which is rejected as a result of the cutoff values, to store the reject rate in the storage unit, to exclude rejected portions of the wavelength bands set by the band-setting unit which have an intensity lower than or equal to the respective certain values stored in the storage unit, to calculate an extent to which distributions of the intensities in the wavelength bands with the respective portions excluded between a first calibration samples and a second calibration sample among the samples are overlapped, and to output to the outputting unit both the reject rate and the extent of overlap. a light source; a flow cell to flow a sample receiving light from the light source and emitting light; a spectral device to diffract the light emitted from the sample; a detection section (680; 104) provided with a plurality of detectors to detect the diffracted light outputted from the spectral device at the respective different wavelength; a signal processing unit to process a signal of the light detected by the detection section; an input unit for inputting an instruction to the signal processing unit; a storage unit to store a result of the processed signal; an outputting unit to display the result; and a band-setting unit to set, via the input unit, wavelength bands to be detected by the detectors;