1. An image generator comprising: an overhead view image generation section for generating an overhead view image by performing a projective transformation, with a virtual viewpoint above a vehicle, of an image captured by an on-board camera for capturing an image of a surrounding region of the vehicle; a three-dimensional object detection section for recognizing a three-dimensional object present in the surrounding region and outputting three-dimensional object attribute information showing an attribute of the three-dimensional object; and an image composition section for generating a monitor display image for vehicle driving assistance by performing image composition of a grounding plane mark showing a grounding location of the three-dimensional object with a portion at the grounding location in the overhead view image, based on the three-dimensional object attribute information.