Automatic pH measurement system was developed by using a flow-through type differential pH-FET sensor. The system was operated by repeating 60 sec cycle consisting of 36 sec of washing period, 4 sec of moving period, and 20 sec of measuring period. In washing period, the differential pH-FET sensor was washed with baseline solution. In measuring period, about 50 μl of sample solution was withdrawn to the space around a measurement pH-FET, and the change of differential source potential was measured. Absolute values of error for nine sample solutions with weak buffering ability were less than 0.146 pH, and were 0.071 pH in average. From this experiment, it was suggested that high-speed and automated pH measurement will be possible by using pH-FET sensors even for sample solutions of weak buffer ability.