To provide a platemaking method used for tandem offset color printing by which patterns in colors printed in the respective printing stations are easily registered among the colors responding to the fan-out of each station even when the fan-out amount differs by the printing station of each color.

The platemaking method for drawing raster data 32 from image data on a plate surface of a printing plate having a plate surface and a non-plate surface is characterized in that the interval of lines in the sub-scanning direction which is orthogonal to the main scanning direction as the drawing line direction of the raster data 32 and is parallel to the axis of a plate cylinder is controlled according to the measured fan-out amount so as to draw an image in a gripping side as elongated in the sub-scanning direction.