1. An image processing apparatus comprising: a processor comprising hardware; and a memory storing computer-readable instructions therein, the computer-readable instructions, when executed by the processor, causing the image processing apparatus to perform: acquiring a first scan image data representing a first scan image having a plurality of pixels and a second scan image data representing a second scan image, each of the plurality of pixels having a pixel value, the first scan image data being acquired by scanning a first part of a specific document, the second scan image data being acquired by scanning a second part of the specific document; determining a reference region that is a part region of the first scan image; calculating, for each of at least one of candidate regions among a plurality of candidate regions in the second scan image, a degree of similarity between the reference region and the each of at least one candidate regions; identifying a corresponding region from among the plurality of candidate regions based on at least one degree of similarity; and generating combined image data by using the first scan image data and the second scan image data, the combined image data representing a combined image in which the first scan image is combined with the second scan image by overlapping the reference region with the identified corresponding region, wherein a size of the reference region is determined based on a variation in the pixel value among the plurality of pixels in the first scan image data, wherein the determining the reference region further includes: setting a plurality of unit regions in the first scan image; calculating a variation value indicating a variation in the pixel value among pixels in each of the plurality of unit regions; and selecting at least one unit region among the plurality of unit regions, each of the at least one unit region having a variation value larger than a threshold value, wherein the reference region is determined based on the selected at least one unit region.