A method by which a halogen atom of a halogen compound can be efficiently replaced with an electrophilic group.Also provided are: a reagent for converting a functional group through a halogen-metal exchange reaction, characterized by comprising either a mixture of a magnesium compound represented by the formula R.sup.1--Mg--X (I) (wherein R.sup.1 represents a halogen atom or an optionally substituted hydrocarbon residue; and X.sup.1 represents a halogen atom) and an organolithium compound represented by the formula R.sup.2--Li (II)(wherein R.sup.2 represents an optionally substituted hydrocarbon residue) or a product of the reaction of the magnesium compound with the organolithium compound; and a process for producing with the reagent a compound in which a halogen atom of a halogen compound has been replaced with an electrophilic group.