An interference checking device includes a contour shape analyzing unit that extracts a contour shape of a tool cross section and a tool length by analyzing a two-dimensional image of a rotating tool, a rotation center analyzing unit that obtains a rotation center of the rotating tool by analyzing the contour shape, a three-dimensional-rotation-shape generating unit that generates a three-dimensional shape of the rotating tool on the basis of the contour shape, the tool length, and the rotation center, and an interference check processing unit that checks whether the rotating tool and a component other than the rotating tool interfere with each other when numerical control machining is performed on a workpiece by using the rotating tool, by using the three-dimensional shape, in which the three-dimensional-rotation-shape generating unit generates the three-dimensional shape by using a left-side contour shape.