An optical writing controller that controls a light source to expose a photoconductor and forms an electrostatic latent image on the photoconductor calculates a correction value for correcting a superimposing position where the developed images for different colors developing each of the electrostatic latent images formed on each of the multiple photoconductors are superimposed based on the detection signal output by a pattern detection sensor that detects a pattern for correcting the superimposing position, controls the multiple light sources to draw a predetermined pattern repeatedly in the sub-scanning direction so that stepwise patterns whose width in the main scanning direction varies with repetition are formed, and determines the width in the main scanning direction of the patterns for correcting based on the strength of the detection signal output by the pattern detection sensor.