1. A printing apparatus which is capable of printing an image on a printing medium by a print head having a nozzle that is capable of creating two or more types of dots varied depending on size, the printing apparatus comprising: a first image data acquiring module that acquires first image data having a plurality of items of pixel data corresponding to an M-ary pixel value (M≧3); a nozzle information storing module that stores nozzle information indicating a property of the nozzle; a nozzle use information setting module that sets whether to use a nozzle corresponding to each item of pixel data for each item of the pixel data of the first image data based on the nozzle information; a second image data creating module that changes to a minimum concentration value a pixel value of pixel data set not to use a nozzle by the nozzle use information setting module in the first image data, and creates second image data that an original pixel value before changed is distributed to a pixel value of an unprocessed pixel located near that pixel value; an N-ary formation process module that performs an N-ary formation process to create third image data, the process being that an M-ary pixel value (M≧3) of pixel data in second image data is transformed to an N-ary value (M>N≧2); a printing data creating module that creates printing data that defines information about dot forming descriptions of the nozzle corresponding to the third image data; and a printing module that prints the image on the medium by the print head based on the printing data.