provided is a method for producing fluoromethane and 3 , 3 , 3 - trifluoro - 2 - propanoyl fluoride 2 chcof ), which are useful as dry etching gases etc ., safely and inexpensively with high purity . according to the method in which 1 , 1 , 3 , 3 , 3 - pentafluoro - 2 - trifluoromethylpropyl methyl ether is pyrolyzed in a gas phase in the presence of a catalyst , the desired fluoromethane and 3 , 3 , 3 - trifluoro - 2 - propanoyl fluoride can be obtained with high selectivity and high conversion of the starting material by a simple process in which a pyrolysis reaction is performed in a gas phase using the inexpensive starting material .