A background generating method of low process cost for use in detecting a moving object based on subtraction process performed between an input image and a base image, and an apparatus with the function of detecting a moving object using its background. The apparatus consists of a feature vector extractor for calculating feature vectors from the input image and an average processor capable of obtaining an average from the calculated feature vectors, diminishing noise occurring for a short time and the influence by the moving object, and forming images that follows the illumination change due to changes in the imaging environment. The images generated by the average processor are accumulated for a predetermined number of frames and the accumulated images are statistically processed by a statistical processor.