Proposed is a method of detecting a moving object, including: providing an image-set at least including a first image and a second image correlated in a time series, the first image preceding the second image; defining a detecting region and a detecting direction so as to construct a virtual gate in the first image; estimating the motion vector in a time series; comparing, by the virtual gate, the second image with the first image so as to determine a difference therebetween in terms of an object's position and motion vector; and retrieving the object to be an effective moving object upon determination of the object as lying within the detecting region defined in the virtual gate and moving in a direction substantively the same with the detecting direction. This invention presents a moving object detection method without the need to construct a background model a priori.