To solve a problem in conventional technology wherein an intake pipe of double structure is applied in a drain spout to guide rainwater free from dust to a rainwater tank, that a constitution to secure flow in the drain spout and/or to release blockage of the drain spout by utilizing the flow of rainwater is not disclosed at all, as it is not intended to secure the flow of rainwater flowing in the drain spout and to release the blockage of drain spout by utilizing the flowing rainwater.

In this fluid coupling mounted in a flow pipe composed of a main body cylinder of double structure comprising upper/lower openings, an intermediate space formed by the double structure is utilized to introduce the air, a circular opening for discharging the air is formed at a lower end of the space, an air intake cylinder communicated with the space and opened to the external is formed on an upper portion of the main body cylinder, and the circular opening for discharging the air, formed on the lower end of the space is opened to an approximately intermediate portion of the main body cylinder.