A flow channel has lateral cross-sectional shapes that are circular in a first section, and rectangular in a second section. The flow channel in the first section is continuously connected to the flow channel in the second section. The rectangular shape has a long/short side ratio of 3 or greater and 7 or less. The second section has a cross-sectional area S2, the first section has a cross-sectional area S1, and S2>S1. There are two first openings, which are open, in one-to-one correspondence, in two side faces of the second section. There are two second openings, one of which is open while extending from one side face of the two side faces to a bottom face of the second section, and the other of which is open while extending from another one side face of the two side faces to the bottom face of the second section.