A passage (3) for a sample solution is formed in a sapphire base body (2) used as a substrate for semiconductor devices. An LED (4) and a photodiode (5) are formed on the base body (2) by a semiconductor manufacturing process so that they face each other across the passage (3). The LED (4) emits light into the base body (2). This light is transmitted through the sample solution in the passage (3), undergoing absorption according to the concentration and other properties of the solution. The transmitted light passes through the base body (2) and reaches the photodiode (5), producing a detection signal corresponding to the incident light amount. Since the light source and photodetector are integrated with the base body (2) serving as a flow cell, the present device is small and lightweight. Furthermore, no cumbersome task of aligning optical axes in the device-assembling process is needed.