There is disclosed an advantageous method for producing a mixed acid anhydride of formula (1):R 1 C(O)OY(O) n (R 2 ) p wherein R 1 , R 2 and Y denote the same as defined below, and n and p denote an integer of 1 or 2, which is characterized by adding a carboxylic acid of formula (2):R 1 COOH wherein R 1 denotes a hydrogen atom, an optionally substituted alkyl group or the like, and an organic base to a solution of a carboxylic acid activating agent of formula (3):(R 2 ) p Y(O) n X wherein R 2 denotes an optionally substituted aliphatic hydrocarbyl group or the like, Y denotes a carbon atom, a phosphorus atom, or a sulfur atom, and X denotes a chlorine atom or the like.