An image processing apparatus includes a reading speed changing unit to change a reading resolution of a manuscript in a vertical direction by changing a carrier linear speed of the manuscript or a moving speed of a reading carriage; a resolution changing unit to change a resolution in the vertical direction of image data of which reading resolution is changed; and a resolution instruction unit to calculate a threshold of a required resolution of an image to determine instruction contents for the reading speed changing unit and the resolution changing unit, based on a data size drawn from a maximum readout size of the manuscript in a main scanning direction and information to specify a pixel and data transfer capability of a data bus, and to provide a resolution for the reading speed changing unit and the resolution changing unit according to the calculated threshold of the required resolution.