A first element and a second element are disposed in postures such that the length directions of the elements are parallel to each other. The first element is disposed in a posture such that a pair of opposing surfaces where a longitudinal flow passage is open is parallel to one main plate. The second element is disposed in a posture such that a pair of opposing surfaces where a longitudinal flow passage is open is parallel to the one main plate. A mixing flow passage formed as a result of the first element and the second element being disposed with a gap therebetween in a facing direction of side plates is provided between a transverse flow passage of the first element and a transverse flow passage of the second element.