This disclosure provides a signal processing device, which includes an echo signal input module for being inputted with echo signals caused by electromagnetic waves emitted from an antenna being reflected on one of more target objects, an echo signal level detecting module for detecting a level of each of the echo signals in association with a distance and an azimuth direction from the antenna, a level change detecting module for detecting a level change between the echo signals from locations which are close to each other where the distances from the antenna are substantially the same but the azimuth directions from the antenna are different, a pattern output module for comparing the level change with a pattern determined in advance to output a level change pattern, and an interference determining module for determining interference based on at least two of the level change patterns.