1. An image analysis device comprising: first and second cameras which together constitute a set of stereo cameras; and a distance calculation section which is adapted to calculate distance to an object in such a manner that a first image of the object is extracted from an image of a field taken by the first camera while a second image of the object corresponding to the first image of the object is extracted from a seeking area being set, in another image of the field taken by the second camera, depending on the extracted first image using a correlation calculation process, and then a parallax between the first and second images is calculated, wherein the distance calculation section is further adapted to set a moving increment based on the width of an object frame that is determined depending on the first image of the object, and to execute the correlation calculation process while moving the first image of the object stepwise at the moving increment in the seeking area so as to extract a new seeking area which consists of a correlation area that exhibits a high degree of correlation with the first image of the object and two areas that sandwich the correlation area.