Schroeder developed a structure to reflect incident sound in a diffuse manner, which is composed of many wells of different depths. This is the so-called Schroeder diffuser. A practically constructed diffuser, which is named QRD (quadratic residue-type diffuser), showed an unexpectedly high sound absorption at low frequency, especially at a frequency lower than the lower design frequency. In this study, the sound field near the surface of the Schroeder diffuser identifies the reason for this high absorption. The particle velocity distributions near the surface of the diffuser and inside the wells are analyzed theoretically and compared with the measured distributions, which were observed by using small particles sprinkled over the observation area. This method encompasses the same idea as the Kundt experiment. The predicted distribution coincides well with the observed one. The observed data show the intense coupled resonance between the wells of different depth. The particle velocity around the edge of the well was extremely large. Computation predicts that the velocity is more than 17 times larger than the velocity of the incident sound wave at the edge. This intense resonance may cause very high absorption. The observed movement of the particle will be presented by VCR.