Just after starting, right and left images, each of which includes a fixed object and does not include an intruding object, are read as background images. Background parallaxes are obtained from the background images. Subsequently, when an intruding object exists in the background images, motion pictures of the intruding object which is moving are read. The brightness of an arbitrary pixel in the right image is compared to the brightness of a pixel, deviated from the pixel by the background parallax, in the left image. As the result of the comparison, the image corresponding to the intruding object is cut from the right image. Consequently, the influences of a three-dimensional object existing in the background and an environmental condition are eliminated. The intruding object can be detected with high accuracy and high reliability.