An object of the present invention is to provide a composition containing fluoromethane having high purity. A method for producing fluoromethane, comprising: pyrolyzing in a gas phase a fluorine-containing methyl ether represented by Formula (1): wherein R1 and R2 are the same or different, and each represents an optionally substituted linear or branched monovalent aliphatic hydrocarbon group, an optionally substituted monovalent aromatic hydrocarbon group, an optionally substituted monovalent cyclic aliphatic hydrocarbon group, hydrogen, or halogen, in the presence of an alumina catalyst to thereby obtain a mixed gas containing fluoromethane and acid fluoride, wherein: the alumina catalyst contains chlorine in an amount of 1.0 wt % or less.