One of the most important characteristics of a ventricular assist device (VAD) is good antithrombogenicity such that the circulating blood does not clot on the surface even when the bypass flow through the device is reduced at the time of weaning-off. A pneumatic and diaphragm-type VAD with excellent antithrombogenicity was developed for clinical use. The pump is made of Japanese-made segmented polyether polyurethane and the maximum output is 7.0 L/min. If the bypass flow was maintained above 2.0 L/min during early postoperative period, thrombus formation was rarely observed even when the flow rate decreased afterwards in chronic animal experiments using 30 goats. Experimental analyses suggested that a biolization mechanism of the material surface covered by absorbed plasma protein might play a major role in the establishment of antithrombogenicity of the pump. No mechanical failure, thrombosis, calcification, and complication in experimental animals occurred when the VAD manufactured under our good quality control system was driven adequately. These results proved that the VAD could be used reliably for at least 3 months. In conclusion, the VAD is safely applicable to clinical cases and contributes to treatments of profound heart failure patients.