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: captured image acquiring means that acquires each of captured images that have been captured by the plurality of imaging units; disturbance level determining means that determines a disturbance level for each of the plurality of captured images that have been acquired, the disturbance level indicating: whether or not a disturbance is present in the captured image; or an extent of the disturbance; captured image selecting means that selects, 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 image generating means that generates a composite image based on the plurality of captured images, and generates the composite image by using the captured image selected based on the disturbance level in the overlapping area.