To provide a road marking image processing device which performs highly accurate road marking recognition by creating a high-quality synthesized bird's-eye image that has no influence from the camera parameter error and noise such as the reflection light from the road surface, lens extraneous matter, shadow of the own vehicle, and the like. A new synthesized range identifying module creates a bird's-eye image of only an area not contained in a synthesized bird's-eye image created based on the road images up to the one captured previously from the current road image. An image synthesizing module synthesizes the bird's-eye image and the synthesized bird's-eye image created based on the road images up to the one captured previously to create a new synthesized bird's-eye image.