PURPOSE: To provide a blocking device which makes operator's operation efficient by automatically changing the shape of an arranged pattern piece in conformity with the shape of texture, etc., to be cut and makes possible even an operator who does not have knowledge about pattern piece drawing to easily perform operation.

CONSTITUTION: When the material is a sheet type, data are corrected S18 into a shape of a mirror image unfolded along the line 30 of a shape 'wa' in Japanese syllabary when a master pattern piece is in the shape 'wa' S16:Yes; a mirror image copy is taken when the master pattern piece is in mirror image relation S20:Yes, and a pattern piece is newly generated S22 and displayed S24. When the material is folded into two or a tube type, a mirror image copy is taken when the master pattern piece is in the shape 'wa' and in mirror image relation S26 and S28:Yes to generate a new pattern piece, but when not, the master pattern piece is displayed S24 as it is.