Abstract: In an automatic analyzer, an influence due to a disturbance component on a measurement result can be appropriately prevented. An automatic analyzer 1 includes: a first light source 102 configured to emit light toward a sample 44; a drive circuit 101 configured to supply a first drive current I3 whose frequency changes from f1 to f2 intermittently or continuously to the first light source 102; a light receiver 113 configured to output a light detection signal IR based on the light transmitted through the sample 44; and a signal processing circuit 111 configured to demodulate the light detection signal IR in accordance with the frequency f1 to f2 of the first drive current I3 and output a measurement signal VL based on a demodulation result.