This disclosure provides a signal processing device, which includes an echo signal input unit for being inputted with echo signals caused by electromagnetic waves discharged from an antenna and reflected on one or more target objects, an echo signal level detector for detecting a level of each of the echo signals with reference to an azimuth and a distance to the antenna, a level change detector for detecting a level change between the echo signals from locations close to each other, the locations of the echo signals being such that the distances from the antenna are substantially the same but the azimuths are different, a pattern output module for comparing the level change with a predetermined reference pattern and outputting a level change pattern, and a missing determining module for determining a missing of a signal based on at least two of the level change patterns.