1. An obstacle alert device comprising: a photographed image acquisition section acquiring a photographed image photographing a scene in the periphery of the vehicle; a photographed-image-of-interest generation section generating a photographed image of interest based on the photographed image; a masked region setting section setting a masked region making undisplayed at least a portion of the scene of the vehicle periphery in the photographed image of interest; an object presence determination section determining whether an object is present or not in an outside region outside the photographed image of interest; and a masked-region highlighted-displaying section causing the masked region to be displayed with highlight when an object in the outside region has entered a region corresponding to the photographed image of interest.