1. An image forming apparatus that forms an image on a recording medium, based on image signals on which add-on signals are added, wherein the add-on signals being adapted to form add-on dots read by an analysis device that analyzes tracking information, comprising: a halftone processing section that executes halftone processing having a halftone processing period, to input image signals; a signal add-on section that adds the add-on signals on the input image signals subjected to the halftone processing by the halftone processing section; a position information acquisition section that acquires position information to determine a relative position of halftone dots by the halftone processing and the add-on signals in a first image area, the first image area is based on a common multiple of the halftone processing period and a minimum interval of the add-on dots formed by the add-on signals, and the position information preventing the add-on signals added by the signal add-on section from crossing over the halftone dots generated by the halftone processing section; and an add-on control section that continues or repeats add-on control processing of the add-on signals, executed by the signal add-on section based on the acquired position information, in a second image area which is based on the common multiple and consecutive from the first image area.