An image forming apparatus includes a phase calculation portion, a pattern recognition portion, and a correction coefficient output portion. The phase calculation portion calculates a shift amount of the scanning lines as phase information and, from the phase information, outputs a first set of coefficients. The pattern recognition portion outputs a pattern concordance rate by performing pattern recognition on the group of pixels of interest and peripheral pixels thereof. The correction coefficient output portion outputs a second set of coefficients for correcting the first set of coefficients in accordance with the pattern concordance rate. An interpolation processing portion performs interpolation calculations by referring to the first set of coefficients, the second set of coefficients, and the pattern concordance rate.