A flow channel structure for causing a first fluid and a second fluid to flow so as to mix together, comprising a substrate in which a flow channel for causing the first fluid and the second fluid to flow is formed, wherein: the flow channel includes a main channel, a plurality of sub channels (14, 18, 22) that are formed on the back surface of said substrate facing the opposite direction to the top surface of said substrate, and causes the second fluid to flow along the back surface of said substrate, and a plurality of merging portions (16, 20, 24) that penetrate said substrate in the thickness direction of said substrate from the top surface side to the back surface side, and merge the second fluid flowing through the plurality of sub channels (14, 18, 22) with the first fluid flowing through the main channel; each of the sub channels (14, 18, 22) of the flow channel has an inlet (14b, 18b, 22b) for introducing the second fluid into the sub channel (14, 18, 22), and the respective inlets (14b, 18b, 22b) are provided independently of one another; the inlet (14b, 18b, 22b) and an opposite end of each of the sub channels (14, 18, 22) are arranged so as to overlap the main channel at respective positions spaced apart along a direction in which the main channel extends when viewed from a direction perpendicular to the top surface of said substrate; and each of the merging portions (16, 20, 24) is formed at a position where the main channel and said opposite end of each of the sub channels (14, 18, 22) overlap in said substrate, communicates between the main channel and said opposite end of the sub channel (14, 18, 22), thereby changing the flow direction of the second fluid flowing through the sub channel (14, 18, 22) connected to the merging portion (16, 20, 24) to the thickness direction of said substrate, and merges the second fluid with the first fluid flowing through the main channel, characterized in that the flow channel is formed on a top surface and a back surface of said substrate, and the main channel causes the first fluid to flow along the top surface of said substrate, the main channel and the sub channels (14, 18, 22) are configured such that the second fluid from one of the sub channels (14, 18, 22) is merged only with said main channel. the flow channel includes a main channel, a plurality of sub channels (14, 18, 22) that are formed on the back surface of said substrate facing the opposite direction to the top surface of said substrate, and causes the second fluid to flow along the back surface of said substrate, and a plurality of merging portions (16, 20, 24) that penetrate said substrate in the thickness direction of said substrate from the top surface side to the back surface side, and merge the second fluid flowing through the plurality of sub channels (14, 18, 22) with the first fluid flowing through the main channel; each of the sub channels (14, 18, 22) of the flow channel has an inlet (14b, 18b, 22b) for introducing the second fluid into the sub channel (14, 18, 22), and the respective inlets (14b, 18b, 22b) are provided independently of one another; the inlet (14b, 18b, 22b) and an opposite end of each of the sub channels (14, 18, 22) are arranged so as to overlap the main channel at respective positions spaced apart along a direction in which the main channel extends when viewed from a direction perpendicular to the top surface of said substrate; and each of the merging portions (16, 20, 24) is formed at a position where the main channel and said opposite end of each of the sub channels (14, 18, 22) overlap in said substrate, communicates between the main channel and said opposite end of the sub channel (14, 18, 22), thereby changing the flow direction of the second fluid flowing through the sub channel (14, 18, 22) connected to the merging portion (16, 20, 24) to the thickness direction of said substrate, and merges the second fluid with the first fluid flowing through the main channel, characterized in that the flow channel is formed on a top surface and a back surface of said substrate, and the main channel causes the first fluid to flow along the top surface of said substrate, the main channel and the sub channels (14, 18, 22) are configured such that the second fluid from one of the sub channels (14, 18, 22) is merged only with said main channel.