To promote mixture of fluids on a plurality of stages, flow channels include a plurality of merging portions which penetrate from a top surface to a back surface of a substrate. An end of each of the sub channels is disposed so as to overlap the main channel at each of positions separated along the direction in which the main channel extends, and each of the merging portions communicates the main channels and the ends of the sub channels with each other, thereby changing a flow direction of the second fluid flowing through the sub channels to the thickness direction of the substrate, and merges the second fluid with the first fluid flowing through the main channels.