This paper presents evaluation of venous return promoting effects of our developed "parallel link type human ankle rehabilitation assistive device (PHARAD)" that can measure accurately foot motions in six degrees of freedom (6-DOF) and reproduce ankle joint motions. The PHARAD performs passive exercise by controlling the foot plate in 6-DOF. The passive exercise can be performed for not only rehabilitation but also prevention of deep vein thrombosis (DVT). In this study, we measured concentration of deoxyhemoglobin (Deoxy-Hb) using multi-channel near infra-red spectroscopy (NIRS)-based tissue oximeters and calculated a gradient of Deoxy-Hb during the venous occlusion. We defined the gradient as the blood flow of vein (BF_<vein>) and evaluated venous return promoting effects of the PHARAD comparing BF_<vein> in resting with those in passive exercise. Experimental results show that there are significant differences of BF_<vein> in resting and passive exercise and this indicates PHARAD has a potential to prevent and decrease DVT.