1. An image processing apparatus that outputs an image formed by combining a background image and an area extracted from an image obtained by reading a recording sheet, the image processing apparatus comprising: a printing unit configured to print an image on a recording sheet; a reading unit configured to read a recording sheet; a converting unit configured to convert the background image into a reference image having the same content as that of the background image but having luminance higher than that of the background image; a print control unit configured to cause the printing unit to print, on a recording sheet, a first image as an entry area to be written in by a user, the first image being a composite image formed by superimposing a fixed image on the reference image obtained by the converting unit; an extracting unit configured to extract, from a second image, an image of an area having luminance lower than a predetermined threshold value, the second image corresponding to the entry area contained in an image obtained when the reading unit reads the recording sheet having the entry area that is written in; an obtaining unit configured to obtain mask data for extracting an image of an area corresponding to the fixed image; and an output unit configured to output a composite image formed by superimposing, on the background image, the image extracted from the second image on the basis of the extracting unit and the mask data obtained by the obtaining unit.