How diffuse radial extracorporeal shock wave therapy (rESWT) affected the flexibility of the infrapatellar fat pad (IFP) was investigated. [Participants and Methods] Eight patients (eight knee) complaining of anterior knee pain participated in this study. The thickness of IFP superficial part before and after rESWT irradiation was measured by ultrasound echo and changes in pain were investigated. [Results] The thickness of the IFP superficial layer at 90°knee flexion changed significantly from 7.7 (7.1-8.9) mm to 13.6 (11.3-14.6) mm on the affected side 4 weeks after rESWT irradiation. Pain on a 100mm Visual Analog Scale (VAS) was 71.5 (65.6-84.8) before treatment, and 12.5 (7.5-23.3) after 4 weeks of treatment, a significant difference. [Conclusion] The results suggest that rESWT irradiation for IFP may contribute to an increase in the thickness of the IFP superficial layer and pain relief.