An image forming device capable of providing an image of excellent quality without unevenness of density. According to the image forming device, a controller measures light quantity for each light emitting element of the exposing device as to the pattern where only one of the light emitting elements is turned on, and as to the pattern where all the light emitting elements are turned on. Based on the data about the measured light quantity, a change rate in the light quantity distribution of the exposing device is calculated. Based on the data about the measured light quantity and the change rate in the light quantity distribution calculated, correction value for each of the light emitting elements of the exposing device is determined.