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A miniaturized analysis system for on-line monitoring of pO2, pCO2, pH was constructed by stacking a reference electrode module, a sensing module, and a sampling module. The liquid-junction reference electrode module used durable thin-film Ag/AgCl electrodes to use an internal solution saturated with KCl. A Clark-type pO2 sensor, a Severinghaus-type pCO2 sensor, and an indicator electrode for a pH sensor were integrated on a sensing module. The sampling module used reversible growth or shrinkage of a hydrogen bubble. The volume change of the gas was converted to the movement of the solution in a flow channel by a silicone rubber diaphragm. The responses of the three sensors were checked by introducing external standard solutions into the flow channel by the sampling module. Clear responses and linear calibration curves were observed for the respective sensors.