There are provided a pair of rollers (W, W1; W, W2; W2, W3; W3, W4) for taking a sheet in a buckle zones (b1-b4), and a pair of rollers (W, W2; W2, W3; W3, W4; W4, W5) for folding a portion of the sheet deflecting from the buckle zones (b1-b4). A rotating shaft of at least one of the rollers of each of the roller pairs is carried by one end of a lever (6). The lever (6) is arranged for swing around a supporting point (8), and the other end of the lever (6) is provided with a spring resiliently biasing the roller carried by the lever (6) in a direction in which the roller is pressed against the counter roller. One end of an adjustment rod (15a-15e) is brought into contact with the other end of the lever (6). The adjustment rod (15a-15e) is guided by a guide (14) in a direction of its axis. The other end of the adjustment rod (15a-15e) is provided with a pulse motor (M1-M5) moving the adjustment rod (15a-15e) through the guide (14). The pulse motor (M1-M5)is driven, whereby the adjustment rod (15a-15e) is moved so as to swing the lever (6), and the gap length between the opposed rollers is adjusted.