1. An image generating device that generates a composite image, in which a plurality of partial images respectively cut out from a plurality of captured images are continuously arranged, using the plurality of captured images respectively having photographic subject regions partially overlapping with each other, the image generating device comprising: a correction device, using a processor, that corrects each target partial image by setting at least one of a first adjacent partial image and a second adjacent partial image, which are the partial images adjacent to the target partial image on both sides in the composite image, as a reference partial image, when at least one of the plurality of partial images is defined as a reference partial image, and the partial images other than the reference partial image are defined as target partial images, wherein the plurality of partial images are continuously arranged in an annular shape; a generation device, using the processor, that generates a first conversion table as a concentration conversion table for performing a correction to approximate a concentration characteristic of each target partial image to a concentration characteristic of the first adjacent partial image and a second conversion table as a concentration conversion table for performing a correction to approximate the concentration characteristic of each target partial image to a concentration characteristic of the second adjacent partial image; and a mixture conversion device, using the processor, that converts a concentration of each target partial image using each of the first conversion table and the second conversion table, wherein: the generation device generates the concentration conversion table as the first conversion table for performing the correction to approximate a concentration characteristic of each target captured image in a first common region to a concentration characteristic of a first adjacent captured image in the first common region when a photographic subject region of the target captured image, which is a captured image providing an origin of the target partial image, and a photographic subject region of the first adjacent captured image, which is a captured image providing an origin of the first adjacent partial image, are overlapped in the first common region; the generation device generates the concentration conversion table as the second conversion table for performing the correction to approximate a concentration characteristic of each target captured image in a second common region to a concentration characteristic of a second adjacent captured image in the second common region when the photographic subject region of the target captured image and a photographic subject region of the second adjacent captured image, which is a captured image providing an origin of the second adjacent partial image, are overlapped in the second common region; and the mixture conversion device converts the concentration of the target partial image to make an influence of the first conversion table larger as compared with an influence of the second conversion table as a position of a pixel in the target partial image is closer to the first adjacent partial image, and to make the influence of the second conversion table larger as compared with the influence of the first conversion table as the position of the pixel in the target partial image is closer to the second adjacent partial image.