Lower back pain is a major health concern worldwide. One cause of lower back pain is the burden on the lumbar region caused by the handling of heavy objects. To reduce this burden, the Ministry of Health, Labour and Welfare in Japan has recommended “squat lifting.” However, this technique, which supports a large force on lower limbs, is not very popular. Therefore, we aimed to develop a power assist suit for squat lifting. In this paper, we propose a gastrocnemius-reinforcing mechanism. Next, we discuss estimation of joint torque from motion analysis of squat lifting in order to construct a prototype. Finally, we describe the performance of the prototype mounted on a human body. The %MVC of the gastrocnemius while performing squat lifting was reduced by 40% using the prototype assist suit compared with the value without using the suit.