An image processing apparatus includes an image data acquisition unit to acquire image data by optically reading a document. A frequency distribution acquisition unit acquires a first frequency distribution of luminances from the image data acquired by the image data acquisition unit. A generation unit generates a second frequency distribution of luminances of a background of the document, by using a maximum frequency of the first frequency distribution as a maximum frequency of the second frequency distribution of luminances of the background of the document. An acquisition unit acquires information indicating a degree of deviation of the second frequency distribution from the first frequency distribution. A correction unit corrects the second frequency distribution based on the information indicating the degree of deviation acquired by the acquisition unit.