PURPOSE: To detect a specific gas with a piece of gas sensor by using the differences in the response speeds of the gas sensor to the respective gaseous components in a gaseous mixture.

CONSTITUTION: The gas sensor 2 has a heater and a thin SnO2 film and has the response speeds varying with the gas to be detected in the gaseous mixture and the other gases. The breathing air is sucked by a specific delivery pump 12 via a detector 10 from a mouthpiece 8 when the breathing air is blown into the sensor. The output of the gas sensor 20 is inputted via an A/D converter 14 to a differentiation circuit 16 and a distributor 20. A memory 22 stores the output V0 before the start of contact with the breathing air, the output V1 of the 2nd second after the start of the contact, the output V2 of the 5th second, and the output V3 of the 10th second in the sampling period of a timer 18. An arithmetic circuit 24 for detecting the concn. of CH3SH (concn. of bad breath) determines to CH3SH from V1-n(V3-V2) and divides the same by the output V0 in the air, thus making the detection based on the output change from the inside of the air. In the formula, (n) is an integer.