A round hole machining method and a round-hole machining device in which machining can be accurately performed regardless of the wear of an edge tool. A non-round hole machining device includes: a machining load data acquisition section for obtaining machining load data corresponding to a machining load acting on the cutting edge of a cutting tool when boring a bore in a cylinder block; a machined shape prediction section for predicting a machined shape of the workpiece by utilizing the obtained machining load data and elastic deformation amounts at respective positions on the workpiece; and a motor control section for machining the workpiece so as to form an inverted shape, that is, a shape formed by inverting the predicted machined shape with respect to a target shape.