Document processing apparatus with image scanning resolution correction function and method thereof

An embodiment of a document processing apparatus that has a function of correcting image scanning resolution according to the present invention includes a first carriage mounted with a first reflector that reflects reflected light from a document fixed on an original placing glass plate in a predetermined direction and a second carriage mounted with a second reflector that changes a direction of incident light from the first reflector by 180°. A correction distance from a default position of the second carriage with which predetermined resolution is obtained in plural scanning positions of the first carriage is calculated. An operation of the second carriage is controlled based on the correction distance obtained.