1. An object detection method for detecting an object in a video image, comprising the steps of: calculating a time-domain average value of each pixel in the image; calculating a variance or standard deviation of each pixel in the image using a time-domain filter with a time constant that is variable for each pixel; calculating a time-domain maximum value of the variance or standard deviation of each pixel in the image; binarizing a current image with a threshold value based on a value obtained by multiplying the maximum value with a predetermined coefficient, for each pixel in the image; labeling the binarized image and treating the thus found plurality of connected areas as pre-detection zones; controlling the variable time-constant depending on whether the pixel is classified into a background or an object, for each pixel in the image; calculating geometric attributes in real space of the plurality of pre-detection zones, and screening the pre-detection zones based on the geometric attributes; carrying out spatial filtering including at least one of skeleton analysis processing, object mask processing, morphology operations, and section analysis processings to the binarized image or an image derived from the binarization; recording a pre-detection zone, which has been subjected to the spatial filtering step or the screening step, as a tracking zone, and updating the recorded tracking zone in accordance with the degree of coincidence with a stored past tracking zone or tracking a temporal positional-change of a tracking zone of interest by extracting a line component in time and space; grouping neighboring tracking zones into a cluster based on a predetermined rule wherein the cluster is created as a rectangular area that contains the neighboring tracking zones and maximum allowable interval between tracking zones integrated into a cluster is defined by a clustering factor; and determining the cluster based on a size of the cluster or a plurality of conditions specifying at least one of a variation in a relative position with a predetermined monitor area and a variation in a relative position with other cluster, wherein the processing is performed by a computer device comprising a processor.