PROBLEM TO BE SOLVED: To provide a drainage system for hot water used in the internal hot water disinfection of a dialyzer capable of efficiently performing drainage without changing a drain pipe to a heat-resistant drain pipe.SOLUTION: This hot water drainage system includes the dialyzer 1, a hot water supply passage 2 for supplying hot water into the dialysing fluid circuit 11 of the dialyzer 1, a drain passage 5 for discharging the hot water from the dialysing fluid circuit 11, a circulating circuit 14 for circulating the hot water supplied to the dialysing fluid circuit 11, the heat exchanger 3 provided to the circulating circuit 14 to cool the hot water supplied to the dialysing fluid circuit 11 and the temperature sensor 4 provided to the drain passage 5. Drainage is stopped when the drainage temperature rises to a set value or above after the start of the supply of the hot water to circulate the hot water in the dialysing fluid circuit 11 and, after hot water disinfection is completed, cooling water is allowed to flow to the heat exchanger 3 to cool the hot water in the dialysing fluid circuit 11 and, when the hot water is cooled to a predetermined temperature or below, drainage is started.
