1. A road surface detection device that detects a road surface region based on disparity information obtained from a captured image of an area around a vehicle that is captured by an onboard camera, comprising: a path estimating portion that estimates a travel path where the vehicle will travel, based on driving information of the vehicle; and a detecting portion that weights the disparity information, and detects a road surface region in the captured image based on the weighted disparity information, wherein when the detecting portion weights the disparity information, the detecting portion gives less weight to the disparity information positioned on a side away from the travel path in the captured image than to the disparity information positioned on a side near the travel path in the captured image.