When the apparatus is energized or a validation check is performed, a deuterium lamp 10 is turned on, and an output value resulting from an A/D conversion of the detection signal for light with a wavelength of 235 nm is obtained. A data processor 22 compares the output value with an upper limit value, which was determined and stored beforehand in a memory 24 (normally, in the manufacturing stage of the apparatus). If the output value is not less than the upper limit, a controller 25 operates an aperture driver 31 to reduce the light quantity by means of an adjustable aperture 30. This operation prevents an excessive quantity of light from reaching a photodetector 19, so that the measurement without absorption by a sample solution 18 can be correctly performed, and the absorbance by the sample solution 18 can be correctly calculated.