1. A scanning printing apparatus for printing an image composed of a plurality of row images, in accordance with image signals, each row image being printed while moving in one direction or its opposite direction on a recording medium, comprising: moving means for moving said scanning printing apparatus itself on the recording medium in said one direction or its opposite direction; a first ejection head for printing each row image of the image on the recording medium, having: a plurality of nozzles for ejecting ink, disposed in other direction orthogonal to said one direction; and ink ejecting means disposed corresponding to said plurality of nozzles and modulated and driven in accordance with the image signals; a second ejection head for printing predetermined invisible marks which cannot be identified by humans on the recording medium; a first sensor for detecting said predetermined invisible marks which have been printed on the recording medium; and control means for controlling said moving means based on detection results of the first sensor.