A reaction mixture obtained by reacting a substrate in the presence of an imide compound having an imide unit represented by the formula (1) can be efficiently separated through crystallization and/or extraction with a specific solvent into the imide compound and a reaction product. (In the formula, X represents oxygen, hydroxyl, or acyloxy.) Furthermore, a mixture comprising the imide compound and a metal catalyst can be efficiently separated through crystallization, adsorption, and/or extraction into the imide compound and the metal catalyst.