1. An object detection device which detects an object that has emerged within a monitoring region, on a basis of an input image from a camera photographing the monitoring region, the object detection device comprising: a preprocessor that corrects an input image by enhancing a contrast of an input image, and adjusts an enhancing amount in the contrast of the input image, wherein the enhancing amount increases with increasing a learning rate of a background image in an object-detecting processor; wherein the object-detecting processor: detects an object that has emerged within a monitoring region by using the input image corrected by the preprocessor and correction characteristic information, the correction characteristic information including an image characteristic derived from the correction in the preprocessor, updates the background image each time the input image is captured, the background image being compared with the input image in detecting the object, and outputs a location information of the detected object to the preprocessor; and wherein the preprocessor further: generates the corrected input image by enhancing the contrast of a whole or a part of the input image by using a converting equation based on a cumulative frequency for each luminance level of the input image, outputs the correction characteristic information to the object-detecting processor, and performs another correction to a subsequent input image within a region in which the object was detected, according to the location information.