A method for a monitoring apparatus using a radar and a camera to detect an object. The radar and the camera each have its own detection area and the method is for detecting and correcting the displacement of each detection area by using a single target having dark and bright parts formed in a specified pattern on its surface and placing it in front of the apparatus where the two detection areas overlap. The direction and orientation of the detection area of the radar are measured and corrected and then the detection area of the camera is corrected. The pattern includes an elongated dark part flanked by a pair of bright parts such that quantity of reflected light obtained by scanning the detection surface has a distribution with waveform having two peaks and a valley in between and corrections on the detection areas are made by analyzing their positions.