Proper orthogonal decomposition (POD) analysis of a supersonic jet impinging on an inclined flat plate is presented. We process data of flow fields and acoustic fields obtained in the previous study in which we found that three kinds of aeroacoustic waves are generated; (i) Mach waves from the main jet, (ii) acoustic waves from the impingement region and (iii)Mach waves from the supersonic flow downstream of the impingement region. For the two-dimensional pressure distribution in acoustic fields by a impinging jet, POD analysis with Fourier transformation is conducted. POD results illustrate locations of the acoustic waves (ii) and Mach waves (iii). Also POD results show that acoustic waves (ii) and Mach waves (iii) are observed in the same POD mode. Thus, the result shows that the acoustic wave (ii) and Mach wave (iii) have the strong relation each other.