An operation monitoring device monitors operations of machining devices in a machining line. Information on a workpiece stocker that constitutes the machining line is acquired, and a current workpiece stock number for the workpiece stocker is updated based on the acquired information. The necessity of change of machining speeds of the machining devices constituting the machining line is determined based on the current stock number for the workpiece stocker. Based on the result of the determination, change of the machining speeds is commanded to the machining devices located before and behind the workpiece stocker in the machining line.