1. An object detection method employing an Histograms of Oriented Gradients (HOG) feature (A) representing an intensity gradient of a target image for detecting existence of a target object in the target image based on an HOG feature (B) representing an intensity gradient computed in advance on a sample image capturing the target object, the object detection method comprising the steps of: computing a plurality of the HOG features (B) having different bin numbers for each of a plurality of local areas in the sample image to obtain a feature pattern representing the existence of the target object; constructing a classifier to judge the existence of the target object in the target image based on the feature pattern; and judging the existence of the target object in the target image by the classifier based on a plurality of the HOG features (A) having different bin numbers computed for each of a plurality of local areas in the target image.