PURPOSE:To fold completely blanks to increase the number of the blanks received on a tray or the like in a device for folding the blanks with a roller by detecting the approach of the blank at the upstream side of the roller to wet the folded portion of the blank. CONSTITUTION:In an apparatus provided with a blank folding section B constituted from a plurality of rollers 11-14 and chutes 15, 16 are additionally provided sensors 30 and wetting means 40. And the sensors 30 are provided on both ends of a blank conveying path 17 at the upstream side of said section B to detect that light beams from a lamp 31 provided opposed to the sensor are interrupted by the blank and the blank is sent to said section B. Also, a wetting means 40 is interlocked with the sensor 30 to lift a block 41 so that a brush 42 contacts the blank and water in a tank 44 is supplied through a pipe 43 to wet the blank by the wetted brush 42. Thus, the folding of the blank in the blank folding section B is completed.