When a first liquid fed into a first flow passage 6 moves therein toward a connecting portion 10 due to capillarity, gas in the first flow passage 6 is pushed by the moving first liquid to be exhausted to the external environment via a third flow passage 14 and an external environment communication passage 8, so that the first liquid moves to the end of a fifth flow passage 16, which is formed in the connecting portion 10, on the side of a second flow passage 7 due to capillarity. Then, when a second liquid fed into the second flow passage 7 moves therein toward the connecting portion 10 due to capillarity, gas in the second flow passage 7 is pushed by the moving second liquid to be exhausted to the external environment via a fourth flow passage 15 and the external environment communication passage 8, so that a liquid-liquid interface level between the first and second liquids is formed in the connecting portion 10.