1. A road marking image processing method for recognizing a road marking within a captured image of a road surface, comprising: capturing a road image of a rear side of a traveling path from a vehicle; measuring a movement amount of the vehicle; identifying a road surface range corresponding to the movement amount as a new synthesized range from the road image, and creating a bird's-eye image for the new synthesized range; and shifting position of a synthesized bird's-eye image that is being stored by length corresponding to the movement amount, synthesizing the synthesized bird's-eye image to the bird's-eye image created for the new synthesized range to create a new synthesized bird's-eye image, and outputting the created new synthesized bird's-eye image.