1. An image processing apparatus comprising: a memory that memorizes a plurality of pickup images in which an object has been picked up by sequentially varying an irradiation direction of a light with each of illumination devices; an extracting portion that extracts an area image of an area indicated by a user from each of the pickup images memorized in the memory; a calculating portion that calculates at least one illumination parameter for each area image by one of calculating an amount of edges from each area image extracted by the extracting portion, executing a clustering process to brightness of each area image extracted by the extracting portion, and executing a totaling process of totaling differences in brightness between the area images extracted by the extracting portion; and a generating portion that generates at least one synthetic image by synthesizing the area images extracted by the extracting portion with the at least one illumination parameter, wherein the synthesizing comprises generating the at least one synthetic image by weighing the area images by the at least one illumination parameter, overlapping the weighted area images with each other, and correcting brightness of the overlapped area image with one of a linear transformation function transforming a brightness scale of an image and a function transforming a histogram.