1. An image processor, comprising: an image converter configured to convert a first image into a first projection image projected on a plane and to convert a second image into a second projection image projected on a plane, the first image acquired by imaging a first target region around a vehicle captured with a first vehicle camera, the second image acquired by imaging a second target region captured with a second vehicle camera, wherein the second target region overlaps the first target region; an image synthesizer configured to synthesize an image from the first projection image and the second projection image; a solid object determination unit configured to detect a solid object in the overlapping region from the first projection image and the second projection image; a difference calculator configured to calculate, when the solid object determination unit detects the absence of a solid object, a difference between an image corresponding to the overlapping region within the first projection image, and an image corresponding to the overlapping region within the second projection image; and a detector configured to detect movement of at least the first vehicle camera or the second vehicle camera out of position by comparing between the calculated difference and a threshold value.