PURPOSE: To set a predetemained margin even to recording paper in a non-fixed form by enabling the printing of document data in such a state that the predeitermined margin is provided within a detected printable area by providing a printing area detection means detecting the printable area of document data to recording paper.

CONSTITUTION: At the time of the printing processing due to a document processing device equipped with a thermal transfer printer, at first, the input from a keyboard 4 is read and, when recording paper of a non-fixed form is selected and line feed width or a character interval is set, a carriage is allowed to scan and the coordinates of the part of the recording paper of each line are measured. That is, coordinates wherein the offset part of a recording head 9 and a recording paper sensor 13 is added to a point where the recording paper detection signal from the recording paper sensor 13 changes are recorded while the carriage is moved by one step. This work is repeated until the lower end of the recording head 9 is detected while line feed is successively performed and the printable area of document data is detected. The printing of the document data is performed in such a state that a predetermined margin is provided in the detected printable area.