After total knee arthroplasty（TKA)，the knee joint flexion angle and walking speed during the swing phase tend to decrease due to the activity of the periprosthetic muscles in the early swing phase. Therefore, we attempted an intervention using a gait-assist device that enables knee flexion during the swing phase by detecting the maximum hip extension angle during the anterior swing phase. The participant was an elderly woman whose left swing phase knee joint flexion angle was less than normal at 3 weeks after left TKA. We focused on the muscle activity of the left periprosthetic knee muscles in the early left-swing phase and examined the effectiveness of the device. The intervention was designed as an AB design, with A being the recommended physical therapy for improving walking ability, including independent walking practice, and B being the same physical therapy as A, except that the gait-assist device was driven during independent walking practice. The patient's 10-m walking speed was moderately improved. These results suggested that intervention using a gait-assist device may alter the muscle activity of the left periprosthetic knee muscles and improve walking speed.