One aspect of the present disclosure is a display processing device including an acquisition unit, a synthesis unit, and a display unit. The acquisition unit is configured to acquire a plurality of images captured by a plurality of cameras provided at a plurality of different positions on a vehicle, the captured images having an overlapping part in which imaging areas of the cameras partially overlap with each other. The synthesis unit combines the captured images acquired by the acquisition unit at the overlapping part of the captured images. The display unit displays the image synthesized by the synthesis unit on a display device mounted on the vehicle. The synthesis unit changes a position of a synthetic boundary, which is a boundary used when combining the captured images, as time passes.