Inkjet printing apparatus and method of correcting step shift thereof

An inkjet printing apparatus configured to perform printing by discharging ink droplets onto a print medium, the apparatus comprising:

• a printing head having a plurality of inkjet heads and configured to perform printing by discharging the ink droplets from the plurality of inkjet heads onto the print medium;
• a chart printing device configured to print a step-shift correcting chart with the printing head, the step-shift correcting chart being used for correcting step shift as printing shift of the plurality of inkjet heads in a transportation direction;
• a reading device configured to read the step-shift correcting chart printed on the print medium;
• a print medium-processed portion detecting device configured to detect a processed portion of the print medium formed in a direction orthogonal to the transportation direction of the print medium;
• an uninfluenced step-shift correcting-chart generating device configured to eliminate an influence of the processed portion of the print medium detected by the print medium-processed portion detecting device from the step-shift correcting chart read by the reading device, thereby generating an uninfluenced step-shift correcting chart;
• and
• a shift-amount calculating device configured to calculate a shift amount for correcting the step shift in accordance with the uninfluenced step-shift correcting chart.