A constant fault alarm rate (CFAR) device for a signal detection system is disclosed herein. The CFAR device includes a first signal selection unit and a second signal selection unit. The first signal selection unit receives a last signal of a lagging sorting array and signals of one or more lagging guard cells, selects any one of the last signal of the lagging sorting array and the signals of the one or more lagging guard cell as a test signal based on a received guard cell size, and outputs the test signal. The second signal selection unit receives the test signal and signals of one or more leading guard cells, selects any one of the test signal and the signals of the one or more leading guard cells based on the guard cell size, and transfers this selected signal to the leading sorting array.