PROBLEM TO BE SOLVED: To provide a farmland conversion method for converting farmland from farmland to paddy field by using a water level adjustment system in consideration of soil preparation. SOLUTION: The farmland conversion method of the present invention is a farmland conversion method for converting farmland used as upland into paddy fields, and is an underdrain that is arranged under the farmland and has a plurality of through holes formed on the outer periphery thereof. The underground water level is adjusted by using a pipe, a water bar connected to the underdrain pipe and adjusting the underground water level using the head pressure, and a water level adjustment system provided with a supply unit for supplying water to the farm ground. A step of raising the water level and a step performed after the step of raising the water level, after the step of reducing the seepage water by pressurizing the ground of the field to reduce the seepage water under the field and the step of reducing the seepage water. This step includes a water supply step of supplying water onto the ground using the water level adjusting system. [Selection diagram] FIG. 9
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