PROBLEM TO BE SOLVED: To efficiently separate a lactam in an amide by the coexistence of an amine with an equimolar amount or more of hydrogen chloride. SOLUTION: When the number of mol of an N,N-disubstituted amide such as N,N-dimethylformamide is A, the number of mol of hydrogen chloride is X and the number of mold of a lactam such as ε-caprolactam is B, the hydrogen chloride is made to coexist in a number of mol satisfying the relationship of the formula A