PURPOSE: To improve the reliability of a water quality meter by monitoring the value detected with a dissolved oxygen concn. meter, etc. in a sewage treatment plant, etc., forcibly cleaning a detection part when a set value or above is attained and eliminatin the measurement error by sticking of foreign matter thereto.

CONSTITUTION: A detecting electrode part 6 of the immersion type water quality meter 1 is put into a part to be measured (sewage) and a PID (proportional plus integral plug derivative action) controller is provided to a control box 4 for cleaning to detect the water quality. A detection signal b is outputted from the PID when the foreign matter sticks thereto. A delay signal c delayed for the prescribed time is also outputted. A rinse signal is quickly emitted when the output signal difference (b-c) therebetween is larger than a set value U. Cleaning water is forcibly emitted from a ringing device 2 to the electrode part 6 to remove the foreign matter. Operation is thus made by maintaining the normal process variable (PV). Since the controller having the function to detect the foreign matter and the function to correct the PV in the above- mentioned manner is provided, the reliability of the water quality controller is improved.