In this paper, we design a PI controller for an electromagnet placed inside a molding machine. The eddy currents inside the electromagnet are a spatially-distributed phenomenon that is difficult to capture by using finite-dimensional systems. First, we show that the theoretical model, expressed by partial differential equations, cannot perfectly capture the properties of the actual machine. Second, we design the PI control system via a loop-shaping by directly using the data of the frequency response obtained from the experiment of the machine. The design problem is reduced to a linear programing problem.