The invention provides a whetstone type of scoring device for production of gypsum boards which can accurately and stably form a groove of a constant depth on a sheet of paper for gypsum board liner, and which can prevent wear of a rotary blade and a backing block. The scoring device (11) for a gypsum board production machine (10) has the rotary blade (24) and the backing block (30). The sheet (3) travels under tension over the block (30), which abuts against the lower surface of the sheet. The block has an opening (32) immediately under the blade. The lower surface of the sheet is released downward by the opening. The sheet is ground by a blade edge (24a) while being slightly bent downward within a range of the opening (within an intimate contact area of an angle .beta.).