1. An image processing apparatus for synthesizing a first image including a transparency-process region and a non-transparency region, and a second image, the image processing apparatus comprising: a memory to store a first color value as transparency-process color information useable for performing a transparency-process to the transparency-process region of the first image; and circuitry: to extract the non-transparency region and a part of the transparency-process region adjacent with each other and existing in the first image as a process target region; to compare a color value of the non-transparency region and the first color value of the transparency-process region adjacent with each other and existing in the process target region; to change the transparency-process color information from the first color value to a second color value depending on a comparison result of the color value of the non-transparency region and the first color value of the transparency-process region adjacent with each other and existing in the process target region, the second color value set closer to the color value of the non-transparency region; to apply the second color value to the transparency-process region existing in the process target region; to enlarge an image size of the process target region having the transparency-process region applied with the second color value in view of an image size of the second image; to perform the transparency process to the enlarged process target region based on the second color value; and to superimpose the enlarged process target region having received the transparency process on the second image to output as a superimposed image.