1. A road lane boundary detection system for detecting a road lane boundary provided on a road, from a road image that is a captured image of a road ahead of a vehicle, comprising: a detection region setting unit that sets a certain region in the road image, as a target detection region to be searched for detection of the road lane boundary; and a detecting unit that processes image data in the target detection region set by the detection region setting unit and that detects the road lane boundary, wherein the detection region setting unit includes: an initial detection region setting unit that sets, in the road image, an initial detection region of a certain size which is positioned close to the vehicle in real space and has a length in a width direction of the road, which is equal to or longer than a predetermined value, as the target detection region; a close detection region setting unit that sets a close detection region that is positioned close to the vehicle in real space, and has a length in the road width direction, which is shorter than that of the initial detection region, as the target detection region; and a remote detection region setting unit that sets a remote detection region that is positioned more remote from the vehicle than the close detection region in real space, and has a length in the road width direction, which is shorter than that of the close detection region, as the target detection region. wherein the detecting unit processes image data in the initial detection region if no road lane boundary is detected, so as to detect a road lane boundary, and processes image data in the close detection region and the remote detection region if the road lane boundary is detected, so as to detect the road lane boundary, and the close detection region and the remote detection region have the same size in the real space.