An abnormal action detector is provided for accurately detecting the position of an object together with an abnormal action using a cubic higher-order local auto-correlation feature. The abnormal action detector comprises a computer which generates inter-frame differential data from moving image data, extracts cubic higher-order local auto-correlation feature data on a pixel-by-pixel basis, adds the feature data for pixels within a predetermined range including each of pixels, calculates an index indicative of abnormality of the added feature data with respect to a subspace indicative of a normal action, determines an abnormality when the index is larger than a predetermined value, and outputs the position of a pixel at which the abnormality is determined. The computer further finds a subspace which exhibits a normal action from past feature data in accordance with a principal component analysis approach. The abnormal action detector is capable of determining abnormality on a pixel-by-pixel basis and correctly detecting the position of an object which has shown an abnormal action.