1. An image forming apparatus comprising: a halftone processor that performs a halftone process on an input image signal using a predetermined halftone processing period; an add-on processor that adds an add-on signal into the image signal on which the halftone process is performed, in a selected period; and an image forming unit which forms an image on a recording medium, based on an output of the add-on processor, wherein among period values in a range defined by a predetermined reference and tolerable values, the selected period corresponds to a period value that causes a maximum least common multiple among least common multiples obtained between a value of the predetermined halftone processing period and the period values in the range defined by the predetermined reference and the tolerable values.