The interference monitoring device comprises: an advanced position calculation unit that calculates an advanced position of the tool or the workpiece ahead by a predetermined period of time based on the program; an interference check unit that draws an estimated shape of a machined part of the workpiece after machining based on the advanced position, and makes the interference check based on the estimated shape of the machined part of the workpiece in the drawing; and an uncut region calculation unit that calculates an error between the estimated shape of the machined part of the workpiece in the drawing and an actual shape of the machined part of the workpiece after machining as an uncut region in the estimated shape of the machined part of the workpiece in the drawing. The interference check unit does not make the interference check in the uncut region.