PROBLEM TO BE SOLVED: To provide a method for synthesizing highly sulfated chondroitin sulfates.SOLUTION: The method for synthesizing highly sulfated chondroitin sulfates includes: a step (A1) of forming A structure of a chondroitin sulfate by making sulfotransferase (C4ST-1) act on the chondroitin; a step (A2) of forming E structure of a chondroitin sulfate by making a sulfotransferase (Ga1NAc4S-6ST) act on the A structure; and a step (A3) of forming triS structure of a chondroitin sulfate by making a sulfotransferase UST act on the E structure.