1. An in-vehicle display apparatus in a vehicle, comprising: a virtual image display device that permits a windshield or a combiner to reflect a predetermined light towards an eye of a driver of the vehicle to display a picture, the virtual image display device displaying the picture in a displayable region as a virtual image appearing ahead of the vehicle; a region recognition circuit that recognizes a target plane region in scenery ahead of the vehicle, the target plane region being a continuous region having (i) a flatness equal to or greater than a predetermined threshold flatness and (ii) an area size equal to or greater than a predetermined threshold area size; and an image output circuit that causes the virtual image display device to display the virtual image in the displayable region such that a driver of the vehicle views the virtual image in the target plane region recognized by the region recognition circuit, wherein the in-vehicle display apparatus cooperates with a camera in the vehicle, the camera providing a forward picture that indicates the scenery ahead of the vehicle along with three-dimensional information; wherein the region recognition circuit converts the forward picture into a three-dimensional driver-viewpoint picture, divides the three-dimensional driver-viewpoint picture into a plurality of part-pictures, extracts flat part-pictures each of which has a flatness equal to or greater than the predetermined threshold flatness from the plurality of part-pictures based on the three-dimensional information, connects the flat part-pictures that are continuously adjoining to provide a flat part-pictures set that has a flatness equal to or greater than the predetermined threshold flatness, and designates, as the target plane region, the flat part-pictures set having an area size greater than the predetermined threshold area size; and wherein the region recognition circuit calculates the flatness as a flatness degree, which is a multiplicative inverse of a gap that is formed by two right planes that sandwich a plurality of points in a surface form of each part-picture or each flat part-pictures set based on the three-dimensional information.