A signal processing device, which includes an echo signal input module for inputting echo signals from an antenna discharging electromagnetic waves to a predetermined area and receiving the echo signals reflected on a target object, an echo signal level detection module for detecting a level of each of the echo signals from each location within the predetermined area, a target object detection module for detecting the target object based on the levels of the echo signals, a correlation processing module for performing scan-to-scan correlation processing of a plurality of scans, and a level adjustment module for adjusting the levels of the echo signals after the scan-to-scan correlation processing. The level adjustment module adjusts the levels of the echo signals corresponding to the locations where the target object detection module detects the target object.