PURPOSE: To reduce the operating burden of an operator by extending the dimension auxiliary lines drawn at the both ends of an angular dimension line to a center point respectively with a simple operation and in a proper from.

CONSTITUTION: An extension data generating part 16 generates the dimension auxiliary line data to extend a dimension auxiliary line drawn at the side designated by an auxiliary line designating part 13 for the angular dimension element pointed by an element pointing part 15 up to a center point with designation of a center extension designating part 14. A rewrite control part 18 rewrites the dimension auxiliary line designated by the part 13 into the dimension auxiliary line data that instructs extension of the dimension auxiliary line up to center point set by the part 16 among those angular dimension pointed by the part 15 in regard of the drawing data which are shown on a display 3 and also stored in a memory 17.