5 in the heavy liquid which flows in the first flow path 9 specifically the outer end of the individual second flow path 10 each constitutes a second intake port 14 and each second intake port 14 opens in a region of the side surface of the main body 8 on which the flow separation header 11 is provided that is each of the second flow paths 10 extends in the horizontal direction in the main body 8 from each of the second intake ports 14 the second intake port 14 opens in a semicircular shape similarly to the first intake port 12 and opens toward the inside of the flow separation header 11 to thereby allow the starting material fluid 2 a of the upper layer which is a light liquid introduced inside the flow separation header 11 to be taken in into the second flow paths 10 the inner end of the second flow path 10 constitutes a merging port 15 this merging port 15 opens on a portion midway through the first flow path 9 in the up down direction more precisely on a part above the above described first intake port 12 and below the boundary surface 3 this merging port 15 allows the light liquid taken in into the second flow path 10 to join the.