A fast-responding Severinghaus-type ammonia gas-sensing electrode was developed by using an air gap formed in a flow channel. An electrolyte solution for the ammonia gas-sensing electrode was separated from a sample solution in the flow channel by an air gap maintained with a hydrophobic layer of silicone rubber intercalated between the electrode substrate and the flow channel. When a sample solution is introduced into the flow channel, ammonia dissipates through the air gap into the electrolyte solution. The accompanying pH change was measured with a thin-film pH-indicator electrode. Because of the air gap, the response was significantly faster than the electrodes of the conventional structure. The 90% response time was less than 1 min for an ammonia concentration higher than 1 mM. A linear relationship was observed in the calibration curve, and the lower detection limit was 20 M. The ammonia electrode was used to construct a creatinine-sensing system. When the solution pH was 10.0, the lower detection limit was 50 M, and the 90% response time was 4 min around the detection limit. The influence of endogenous ammonia could be minimized by taking a differential signal with a double indicator electrode structure. The signal originating from creatinine could be differentiated clearly in the presence of ammonia of comparable concentrations.