In an inkjet printer that prints an image by a forward scan and a backward scan of printing heads, it is determined whether or not a color unevenness occurrence value set on the basis of values of unit image data constituting image data is more than a predetermined threshold value. If the color unevenness occurrence value is less than the threshold value, data that enable a dot to be formed in a unit area corresponding to the unit image data by the forward scan and the backward scan of the printing heads are generated. If the color unevenness occurrence value is more than the threshold value, data that enable a dot to be formed in the unit area only by one of the forward scan and the backward scan of the printing heads are generated.