To provide a pH controller which is inexpensive and excellent in the stability and precision of pH values.

This pH controller measures the pH of water to be processed with a pH meter 3 and intermittently adds a pH adjusting agent to the water to be processed according to the measurement result, and is equipped with a memory which stores a target pH value and an arithmetic means which computes addition intervals and the addition continuation of the pH adjusting agent according to the measured pH value and the pH value stored in the memory. A memory of this arithmetic means (controller) 4 is stored with a data table 4a wherein additon/stop times of acids and alkalis corresponding to differences in pH values are set.