An image processing apparatus includes a controller configured to control an image scanner to scan a document sheet and generate target image data, generate an original histogram of an index value representing a luminance of each pixel forming the target image data, determine whether a particular condition is satisfied that includes a requirement that a difference value between an orientation of the document sheet and a reference orientation in the image scanning process is equal to or larger than a particular value, when the particular condition is satisfied, calculating a count of out-of-document pixels other than pixels representing an image of the document sheet, generate an adjusted histogram by subtracting at least a part of the count of the out-of-document pixels from frequencies of index values within a particular range in the original histogram, setting a specific threshold of the index value for binarization processing, based on the adjusted histogram.