There is provided a technique for detecting leakage in extracorporeal blood circulation while suppressing increased cost in a dialysis device. A dialysis device includes an artery-side blood circuit, a dialyzer, and a vein-side blood circuit. The dialysis device detects an abnormality based on a difference between theoretical value and measured value of a blood concentration in the vein-side blood circuit. The theoretical value of the blood concentration in the vein-side blood circuit is specified based on blood concentration and blood flow rate in the artery-side blood circuit and a water removal rate in the dialyzer.