PURPOSE: To improve the operability of a document preparing device by detecting a paragraph having the same format information as that of a leading character string in the paragraph of a target writing in which a margin is to be set up from a writing continued before or after the target writing and setting up detected left margin length in the paragraph of the target writing.

CONSTITUTION: A retrieving means (margin candidate discriminating device) 9 retrieves a paragraph having the same format information as that of the paragraph of a target writing obtained from the comparing means (margin candidate discriminating device) 9 from paragraphs detected by a detecting means (paragraph head extracting device) 4. The margin length determining means (margin candidate discriminating device) 9 reads out the left margin length of the paragraph retrieved by the retrieving means 9 from a left margin reader 6. A left margin setting device 10 sets up the margin length determined by the determining means 9 in the paragraph of the target writing. Since a user can automatically set up a left margin in the target writing only by outputting a margin setting command from an input means (keyboard) 1, the operability of the device can be improved.