PROBLEM TO BE SOLVED: To provide a dialyzer capable of suppressing an increase in an internal pressure of a liquid supply passage when actuating a supply/discharge pump.SOLUTION: A dialyzer 1 includes: first and second chambers 21 and 22 that are partitioned into supply chambers 21a and 22a, recovery chambers 21b and 22b, and variable volume chambers 21c and 22c respectively and a silicone oil pump P3 (a supply/discharge pump) for feeding a liquid to between the variable volume chamber 22c and the variable volume chamber 22c. A liquid supply passage 23 has a second on-off valve V12 (a pressure release valve) for releasing the pressure in the liquid supply passage. One chamber supplies the dialyzer with a fresh dialysate solution and recovers the used dialysate solution and, while the supply/discharge pump feeds the liquid from the variable volume chamber of the one chamber to the variable volume chamber of the other chamber, the internal pressure of the other chamber which increases thereby is released by the second on-off valve. The dialyzer can suppress an increase in an internal pressure of the liquid supply passage during dehydration.
