Direct computations and experiments of a hole-tone feedback system are conducted. The mean velocities of an airjet are 8 and 10 m/s in the computations, 6-13 m/s in the experiments. The diameters of a nozzle and an end plate hole are both 50 mm, and an impingement length between the nozzle and the end plate is 50 mm. The computational results agree well with the experimental data in terms of qualitative vortical structures and a relationship between the most dominant holetone frequency and a jet speed. Based on the computational results of the air-jet speed of 8 m/s, a Proper Orthogonal Decomposition (POD) analysis of the whole pressure fluctuation field is conducted. The 1 st and 2nd POD modes are nearly in anti-phase, and alternatively appearing helical structures are observed upstream of the end plate hole in an isosurface plot of the eigenfunctions of the modes. Dominant behaviors of vortex shedding from the end plate hole are represented by the 3rd and 4th modes. As the result, dominant variation of the pressure fluctuation field is successfully extracted by the present POD analysis.