System data handling in CAD sY8tem for designing control systems is discussed. A man-oriented data description method for a wide sense block diagram and its automatical transformation into the state space description is proposed. This work is a part of CAD system: CADPACS-T which has been under development for designing control systems in our laboratory. The proposal data description for a wide sense block diagram has the following features: 1) to correspond nicety to the block diagram and to be suitable for a man-oriented expression, 2) to express even a large scale system compactly by partitioning into some subsystems defined externally, 3) to be easy to add /or alter the input-output terminals, parameters or elements, and 4) to need not to assign the connection relationship explicitly owing to adopting the input-output terminal/line names. Moreover, system data handling in designing the PI controller for an actual boiler system expressed in a block diagram is taken up as an example.