The device comprises a first rotary plate (21), a number of bending plates (22) mounted on the peripheral edge (21a) of the first rotary plate (21), a number of positive motion cams (23) interposed between the first rotary plate (21) and the bending plates (22) and mounted on the peripheral edge (21a) of the first rotary plate (21), a second rotary plate (44) opposedly disposed on the side opposite to the first rotary plate (21), and a number of auxiliary plates (52) mounted on the peripheral edge (44a) of the second rotary plate (44), it being arranged that the bending plates (22) and the auxiliary plates (52) synchronously enter into a diaper assembly (70) while a guide arm (32) enters into the clearance between guide blades (54), causing the opposite sides (72a) of an inside leg region (72) to be held between the guide arm (32) and the guide blade (54), whereby the inside leg region (72) is bent into the inside of the diaper assembly (70).