1. A road shape recognition device that recognizes a road shape of a road that lies ahead in a travel direction of a vehicle, the road shape recognition device comprising: an image capturing unit that captures an image ahead in the travel direction of the vehicle; a road region detection unit that detects, based on the image captured by the image capturing unit without using a line on the road, a road region of the image which is occupied by the road; and a road shape estimation unit that estimates the road shape of the road based on the road region detected by the road region detection unit, wherein the road region detection unit detects the road region based on at least one of luminance information and color information of the image captured by the image capturing unit, and wherein the road region detection unit obtains information of at least one of luminance and color distributions of a previously detected road region from a past image from which the previously detected road region has been detected by the road region detection unit in the past, and detects the road region of the road from the image based on the obtained information.