The adverse effect on measurement accuracy brought about by the transmission of light beams through portions of a liquid sample with different concentrations, i.e., a concentration distribution in the vertical direction of a container, is prevented by using semiconductor light sources of two different types with different wavelengths. The semiconductor light sources (2, 4) of two different types are housed in the same package (5) such that a detector (9) can capture the light beams emitted by the light sources after their optical axes have intersected with one another. The multiple light beams can be thus caused to pass through portions with substantially the same concentration and therefore can be detected without being influenced by the difference in concentration of the sample in the container.