An automatic image synthesizing apparatus including an image search unit configured to search for and extract a frame image having an object to be learned or replaced from a plurality of frame images inputted through a vehicle camera; an instance object detection unit configured to detect an instance object to be learned or replaced from the extracted frame image; an image extraction and object class information detection unit configured to extract mask-segmented objects from the extracted frame image and form a data-structure; a standard image mapping unit configured to map a standard image matching a class value of the mask-segmented object; and a generative adversarial network (GAN) network unit configured to receive the selected standard image and the mask segmentation image and learn a replacement image or produce a replacement image based on a learning result.