Gas sensing materials for detecting inflammable gas such as alcohol, propane, acetic acid, carbon monoxide, hydrogen were developed by utiliting $MgO-Cr_2O_3-TiO_2$ system. Between 30$0^{circ}C$ and 50$0^{circ}C$, reversible chemisorption becomes dominant and the electrical canduction of P-type semiconductive with the gas chemisorption. The ceramic sensor exhibits a high sensitivity to particular reducing gas such as alcohol, whereas propane and butane have little effect on the resistivity. The time response of adsorption is estimated to be about 20 sec. On the other hand, the desorption process, which corresponds to oxidation due to oxygen adsorption, take more than 60 sec. Thus the ceramic sensor can be used as a alcohol sensor in an ambient aunosphere. As the oxygen concentration is increased from 0.1 to 10 precent($10^3-10^6ppm$), the resistance decreases rapidly but stabilizes at higher concentration.