In order to detect abnormal objects with a low-cost arrangement at high speeds while avoiding unwanted influence of variations of a light source and/or regularly vibrating objects in the environment concerned, there is provided a block data calculation unit 4200 inputted from a camera into blocks, an object candidate extraction unit 4700 which compares image data of a frame to be processed and the immediately preceding frame in units of blocks to thereby extract an abnormal object candidate in accordance with the presence or absence of edges and a longitude-to-lateral edge ratio change rate, and an object judging unit 4800 for determining or "judging" whether the abnormal object candidate is a true abnormal object, wherein it is an abnormal object where movement of the abnormal object candidate is traceable for a prespecified length of time period.