To print the line data of raster format print data accurately on a print object by aligning the mechanical axis of a raster printer mechanically with an axis on the print object.

A table 16 is supported adjustably in the rotational direction and a sensor 12 for reading a mark is provided on the head section 8 side. As the preprocessing of printing, the head section 8 is moved relatively to the table 16 in the direction of XY-axes and the reference mark of a print object on the table 16 is read out at a plurality of points by means of the sensor 12. Based on the well-known data of the reference mark on th print data and the actual data of the reference mark on th print data, inclination of the axis of the print object against the mechanical axis of the raster printer is determined. The table 16 is then turned based on that inclination, thus aligning the axis of the print object with the mechanical axis of the raster printer.