PROBLEM TO BE SOLVED: To provide a drainage device 10 having a triple cylinder structure including an outer cylinder 12, a first inner cylinder 14 and a second inner cylinder 16, and used for a field 100 for executing plow land usage alternation, in which the outer cylinder 12 is formed in an upper end-opening bottomed vertical pipe shape, and a drain water inflow port 20 and a discharge port 22 are formed in its sidewall lower end part, and the first inner cylinder 14 is fitted in the outer cylinder 12, and opens-closes the drain water inflow port 20 of the outer cylinder 12 by vertically moving or rotating in the circumferential direction, and the second inner cylinder 16 is also fitted in the first inner cylinder 14, and changes a height position of a field surface water inflow port 42 formed in its upper end part by vertically moving.SOLUTION: Its water level can be adjusted, and drain water from a deep groove or underground can be discharged, by draining field surface water of a preset water level or more, with a simple constitution.
