To reduce lower back pain of workers, we developed an assist suit for lumbar support in our previous studies. This suit, called AB-Wear II, has mechanisms to aid muscle strength through the inflating power of artificial muscles and balloon actuators, and reduce compressive forces acting on the spine through leaf springs. However, due to the stiffness of the leaf spring and the soft fabric supporter fixed to the wearer's waist, the device slipped out of place during a lifting motion, causing a loss of assistive force. In this study, a new version, AB-Wear III, was developed. The leaf springs in AB-Wear III are composed of a soft material that allows them to follow a wearer's movements. The leaf spring supports the posture of the spine by applying pressure against the back, and it should also reduce the shear force that causes lower back pain. In addition, a lumbar fixation mechanism using a leaf spring wound in an arc shape near the pelvis was developed to fix the device to the body of a wearer and prevent the device from slipping. Based on the electromyography (EMG) of the erector spinae muscles during a lifting motion while wearing AB-Wear III and a questionnaire, the optimal contact position between the back and leaf spring was found to be 60% of the spine length. At the abovementioned length, EMG was reduced by 16.7% than without the device. Subjective experiments also confirmed that the wearers' endurance was improved and lower back pain was reduced using the device.