There is disclosed an advantageous mixed acid anhydride production method of formula (1): R1C(O)OY(O)n(R2)p  (1) wherein R1, R2 and Y denote the same as defined below, n and p denote an integer of 1 or 2, which is characterized by adding a carboxylic acid of formula (2); R1COOH  (2) wherein R1 denotes a hydrogen atom, an optionally substituted alkyl group or the like, an organic base to a solution of a carboxylic acid activating agent of formula (3); (R2)pY(O)nX  (3) wherein R2 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.