1. An image processing apparatus comprising: a normalization unit that generates a plurality of normalized images by correcting, based on different imaging conditions, brightness of a plurality of images at different brightness obtained by imaging a same subject with the different imaging conditions; and an image combining unit that generates a combined image by combining the plurality of normalized images, wherein when the normalized image corresponding to a reference image that is a brightest image among the plurality of images is defined as a reference normalized image, the image combining unit combines the reference normalized image and a non-reference normalized image that is the normalized image other than the reference normalized image to bring gradation of pixel of the combined image close to gradation of pixel of the reference normalized image if the gradation of pixel of the combined image is smaller than the gradation of pixel of the reference normalized image.