1. An image reading apparatus, comprising: a data calculation unit to calculate moving average data and second derivative data for a plurality of samples made of multivalued data, the plurality of samples being data in a vertical direction and a horizontal direction in reading an inputted image data; a position detection unit to detect first and second positions at which the moving average data and second derivative data have a predetermined relationship based on the moving average data and second derivative data, for each of the plurality of samples; a parallel region determination unit to determine a parallel region based on the first and second positions detected for each of the plurality of samples; an inclination detection unit to determine inclinations of four sides of the inputted image based on the determined parallel region; and an inclination detection unit to correct the inputted image data based on the detected inclinations.