PURPOSE: To achieve structural simplification and miniaturization, by using an optical fiber and a converging rod lens in an optical system.

CONSTITUTION: A fluid to be measured flowed into a measuring tank 1 from an inflow pipe 2 is flowed out from an outflow pipe 3 to form steady flow. Lights issued from a light source 7 are passed through an optical fiber 4 and a rod lens 5 to be emitted in a parallel stream and permeated through the fluid to be measured to be incident to a light detecting element 8 through an optical fiber 6. The turbidity of the fluid to be measured is measured from the output signal of said detecting element 8. By this method, structural simplification and miniaturization are achieved.