PROBLEM TO BE SOLVED: To provide a U-shaped ditch made of a synthetic resin which is never floated up by the buoyancy of groundwater even when a groundwater level in a laying location of the U-shaped ditch made of a synthetic resin is high or when the groundwater level in the laying location rises after the U-shaped ditch is laid.SOLUTION: In a U-shaped ditch 10 made of a synthetic resin having an external surface formed in corrugated shapes, a valve structure 20, which does not allow water to flow out from the inside of the U-shaped ditch 10 to the outside thereof, while allows water to flow from the outside of the U-shaped ditch 10 to the inside thereof, is disposed in a flat part 18 formed in a bottom part 12 of the U-shaped ditch 10 on the outside of a connection part between U-shaped ditches 10.