A downstream edge of a first guide plate has a convex shape in a carrying direction such that a width thereof gradually reduces from both lateral sides toward the center. A central portion of the downstream edge is formed with a protrusion, a width of which becomes narrower in the carrying direction. An upstream edge of a second guide plate has a concave shape in a reverse direction to the carrying direction so as to correspond to the shape of the downstream edge of the first guide plate. A central portion of the upstream edge is formed with a cutout, a width of which becomes narrower in the carrying direction. The downstream edge of the first guide plate is adjacent to the upstream edge of the second guide plate, and the protrusion is adjacent to the cutout so as to enter thereinto.