Low back pain is an important international health problem. Factors of back pain have a burden on the lumbar region due to the handling of heavy objects. The Japanese Ministry of Health, Labor and Welfare recommends Squat Lifting as a method with less load on the waist. However, Squat Lifting is not used much because it has a large workload. Therefore, we aimed to develop a power assist suit for spreading Squat Lifting. In this paper, we first describe a device with a knee joint extension mechanism by a leaf spring and its problems in a previous study. Next, we propose a sliding mechanism as a solution. Then, a three-dimensional motion analysis for determining the necessary assist torque during Squat Lifting will be described. Finally, we will explain the leaf spring characteristic test which determines the specification of leaf spring.