An image reading apparatus has a data calculation unit (31,32) to calculate moving average data and second derivative data for a plurality of samples made of multivalued data, a position detection unit (41, 42) to detect first and second positions at which the moving average data and second derivative data have a predetermined relationship, a parallel region determination unit (5) to determine a parallel region based on the first and second positions, an inclination detection unit (61 to 64) to determine inclinations based on the parallel region, and an inclination detection unit (7) to correct image data based on the inclinations.