An integration circuit 10 comprises: an amplifier and an integration capacitance section arranged in parallel to each other between an input terminal and an output terminal; and capacitance value switching means for switching the capacitance value of the integration capacitance section into any one of Cf.sub.1 through Cf.sub.K; and inputs electric charge output from a photodiode PD, so as to accumulate the electric charge into the integration capacitance section, so as to output an integration voltage having a value corresponding to the amount of the accumulated electric charge. An A/D converter circuit 20 comprises reference voltage switching means for switching a reference voltage used in A/D conversion, into any one of V.sub.ref,1 through V.sub.ref,L, and inputs the integration voltage output from the integration circuit 10, so as to A/D-convert the integration voltage on the basis of the reference voltage switched and set to be by the reference voltage switching means, so as to output a digital value corresponding to the integration voltage.