1. An image processing apparatus comprising: a combining processing unit combining plural images into a piece of combined image, the plural images being taken by a plurality of image-capturing from a certain position to different plural directions, wherein the combining processing unit: extracts a parameter by performing block matching processes at individual boundaries in such a manner that boundaries of a plurality of selected images are disposed in overlapping relation, performs, based on the parameter, block matching processes for all boundaries to be combined, evaluates results of the block matching processes with respect to all the boundaries, and performs combining of the images to reduce errors by updating an optical axis direction so as to reduce errors in all the boundaries.