Low back pain affects society and health, and has become an important issue internationally. Among the causes of low back pain are the burden on the waist from handling of heavy objects. The Japanese Ministry of Health, Labor, and Welfare recommends squat lifting as a lifting method with less burden on the waist. However, squat lifting is not performed frequently because of its large workload. Hence, we propose the development of a power-assist suit for the spread of squat lifting. Previously, we developed a device with a knee joint extension mechanism by a leaf spring. However, the leaf springs inhibited the flexion of the knee joint during walking. We propose a power-assist suit for the lower limb with a sliding mechanism to reduce the force by leaf spring during walking. Next, the operation of squat lifting was analyzed, and the target assist torque was set to 14.4 N m. Subsequently, according to the leaf spring characteristic test, the specification of the leaf spring was set to 100 mm in length and five sheets. Finally, a subjective evaluation of the prototype was performed, the burden reduction effect at the time of lifting was improved, and the bending easiness of the knee joint at the time of walking was confirmed.