1. An image processing device, comprising: an extracting unit configured to extract boundaries of a document in a first direction and roughly-detected document corners of the document in a document image captured for the document; a determining unit configured to determine candidate page corners of the document on the boundaries of the document in the first direction around the roughly-detected document corners; and a selecting unit configured to select the candidate page corners with the closest intra-page area pixel feature to a page corner pixel feature of the document among the candidate page corners as document corners of the document, wherein the first direction is a horizontal direction or a vertical direction of the document image.