A first image of two images to be compared for similarity is divided into small areas and one small area is selected for calculating the correlation with a second image using a correlative method. Then, the position difference, luminance ratio and similarity in an area where the similarity, which is the square of the correlation value, reaches its maximum, are found. Values based on the similarity are integrated at a position represented by the position difference and the luminance ratio. Similar processing is performed with respect to all the small areas, and at a peak where the maximum integral value of the similarity is obtained, its magnitude is compared with a threshold value to evaluate the similarity. By extracting the small area voted for that peak, it is possible to extract a similar area.