A LED floodlight, comprising: a main unit that is longitudinally formed by extrusion molding of a metal material and has in one side an opening of a concave groove having a U-shaped lateral section, one or more LED units attached to a central portion, as viewed in said cross-section, of an inner bottom wall defining said concave groove in said main unit, and a power source unit that is attached to a portion of other side except for said one side where the opening of said concave groove of said main unit, a transparent plate that is attached to said opening of said concave groove to cover a front of said LED unit, and an upper lid and a lower lid that close up said longitudinally upper and lower ends of said concave groove in said main unit to isolate said LED unit together with said transparent plate from an environment, wherein said main unit is formed on a back side of said inner bottom wall of said concave groove by said extrusion molding and has one or more ventilating ducts that are parallel with said extrusion molding direction and are open at upper and lower ends, an area having a large heat capacity is provided between said inner bottom wall to which said LED unit is attached and said ventilating duct, characterized in that said main unit is configured such that said LED unit is turned on in a posture where said longitudinal direction of said ventilating duct defines a vertical direction, thereby achieving a chimney effect by which heat conducted from said LED unit is transferred to the airflow going up through said ventilating duct, and wherein said ventilating duct includes a drift means inside for giving a rotation to an airflow going up through said ventilating duct. a main unit that is longitudinally formed by extrusion molding of a metal material and has in one side an opening of a concave groove having a U-shaped lateral section, one or more LED units attached to a central portion, as viewed in said cross-section, of an inner bottom wall defining said concave groove in said main unit, and a power source unit that is attached to a portion of other side except for said one side where the opening of said concave groove of said main unit, a transparent plate that is attached to said opening of said concave groove to cover a front of said LED unit, and an upper lid and a lower lid that close up said longitudinally upper and lower ends of said concave groove in said main unit to isolate said LED unit together with said transparent plate from an environment, wherein said main unit is formed on a back side of said inner bottom wall of said concave groove by said extrusion molding and has one or more ventilating ducts that are parallel with said extrusion molding direction and are open at upper and lower ends, an area having a large heat capacity is provided between said inner bottom wall to which said LED unit is attached and said ventilating duct, characterized in that said main unit is configured such that said LED unit is turned on in a posture where said longitudinal direction of said ventilating duct defines a vertical direction, thereby achieving a chimney effect by which heat conducted from said LED unit is transferred to the airflow going up through said ventilating duct, and wherein said ventilating duct includes a drift means inside for giving a rotation to an airflow going up through said ventilating duct.