1. An object estimation device comprising: smoothing means for generating a plurality of smoothed images of different scales by repeating convolution of an image with a smoothing filter having filtering characteristics corresponding to a contour of an object; differential image generating means for generating a plurality of differential images by calculating a difference between each pair of the smoothed images apart from each other by a predetermined scale interval in the smoothed images generated by the smoothing means; combining means for generating a combined image by combining the differential images generated by the differential image generating means; and position estimating means for estimating a position of the object from a position where a maximum or minimum signal value is found in the combined image generated by the combining means.