1. A composite image generation apparatus that is mounted in an own vehicle and generates a composite image which is a combination of captured images acquired by a plurality of imaging units, the composite image generation apparatus comprising: a processor configured to acquire each of captured images that have been captured by the plurality of imaging units; determine a disturbance level for each of the plurality of captured images that have been acquired, the disturbance level indicating an extent of a disturbance; select, in an overlapping area in which imaging areas of the plurality of captured images overlap, one or more captured images from the plurality of captured images so that the area occupied by a captured image having a higher disturbance level among the plurality captured images is smaller; and generate a composite image based on the plurality of captured images, and generate the composite image by using the captured image selected based on the disturbance level in the overlapping area, wherein the processor is configured to deduct a number of points from an allotted score corresponding to each of the plurality of captured images in response to the disturbance level.