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A new type of semiconductor odor sensor based on indium oxide (In,O-) has been developed. By choosing the material and the surface modification of the semiconductor, we have obtained certain performances as follows : (ῌ) small influence of humidity on the output signal of the sensor ; (῍) short response time ; (῎) good characteristics of selectivity for odor detection, that is, the sensor has not only high sensitivity to odor gases, but has low sensitivity to odorless gases, such as H, and CH.. These characteristics of sensing gases make it possible to measure the odor reliably in practical use.We also present our applications of this sensor, portable instrument which indicate the odor level and the estimated odor index, and stationary instrument for on-line monitoring of ambient odor.