Provided is a gas sensor comprising a gas-sensitive layer consisting of nickel oxide (NiO) doped with chrome (Cr) so as to selectively detect methylbenzene gas. The gas sensor, according to the present invention, has a gas-sensitive layer consisting of Cr-doped NiO. The gas sensor exhibits superior selectivity for methylbenzene gas when compared to other gases. The gas sensor can be easily and mass produced according to the production method of the present invention and is not affected by the microstructures of the material forming the gas-sensitive layer. The present invention relates to a gas sensor exhibits a negligible sensitivity of benzene, formaldehyde, and alcohol and exhibits significantly high selectivity and sensitivity of methylbenzene gas such as xylene and toluene when the NiO sensor material with a hierarchical structure favorable to gas dispersion and response is doped with Cr.