1. An apparatus for monitoring an area adjacent to a subject vehicle, comprising: image taking means for taking an image of an area at and around a place to which the subject vehicle is directed; predicting means for predicting a region to which the subject vehicle will travel, the predicted region having a left-hand boundary and a right-hand boundary; setting means for setting a detection area near at least one of the left-hand boundary and the right-hand boundary of the predicted region, the detection area extending along a direction of travel of the subject vehicle; and deciding means for deciding whether or not a three-dimensional object exists in the detection area in response to the image taken by the image taking means.