To obtain the subject new compound being a reagent for introducing a fluoromethyl group or -CH2F to a carbon atom of carbonyl in an organic substance structure, comprising a specific magnesium fluoromalonate.

This compound is shown by the formula (X is O or S; and R' is a radical capable of being cleft by hydrogenolysis). The compound is obtained by reacting hexafluoropropane with ammonia to form 2,3,3,3- tetrafluoropropionitrile, treating the reaction product with an alcohol solution of a strong base such as an alkali metal alkoxide, etc., to given a fluoromalonic acid dialkyl ester, transesterifying fluoromalonic acid dimethyl ester with an R'-OH in the presence of a strong acid catalyst, then saponifying one of two R' ester groups of an R'-fluoromalonic acid ester, isolating benzylfluoromalonic acid and precipitating magnesium benzylfluoromalonate from a THF solution filtered off by hexane.