PURPOSE: To rapidly and precisely detect pH value of fluid flowing through a pipe at a low cost by adding an indicator into the fluid flowing through the pipe, and by detecting light transmitting through thus obtained mixture while allowing the mixture flowing down in a space.

CONSTITUTION: The pH value detecting part 3 is connected to a fluid pipe line 2 for measuring a pH value of a fluid, branching from a main fluid pipe 1, and a valve 4 for causing the fluid to flow into the pipe line 2 is connected to the pipe line 2 upstream of the detecting part 3. When a valve 7 connected between the pipe line 2 and an indicator feed circuit 6 is opened, a sampled fluid flows through the pipe line 2 whose pH value is to be detected, while an indicator is added to and mixed with the sampled fluid which then flows down, as a liquid stream 9, in a space within a cover 10 from a nozzle 8. Since the hue of the sampled fluid varies depending upon its pH value, the pH value can be obtained from the intensity or hue of light transmitted through or reflected from the liquid stream 9 along an optical path and received by a detector 12.