The scarcity of frequency resources has recently become a serious problem. Dynamic spectrum access to spatial white space is a powerful technique for exploiting frequency resources. To search for the white space, a spectrum database records the measurement results and estimates the propagation loss and shadow effect. When the received signal strength indication (RSSI) to the primary system is calculated, using the estimated propagation loss and the shadow effect, these estimated factors are changed owing to the time transition or unexpected disturbances. To avoid harmful interference to the primary system, caused by the difference between the estimated interference signal and the practical one, this paper proposes a monitoring system with a radio sensor for the interference signal in the spatial white space. In the proposed system, the radio sensor estimates the entire probability distribution of the RSSI from the partially detected RSSI. The Kolmogorov-Smirnov test is subsequently performed on the detected probability distribution of RSSI and the estimated distribution by the spectrum database. From the computer simulation, the proposed monitoring system achieves a high accuracy in detecting changes in the RSSI.