A lumped constant line approximating system capable of approximating a distributed constant line with required accuracy. The system receives inputs of object circuit information and analysis conditions (Steps 310, 320), calculates a product gamma l of the propagation constant gamma and the line length l (Step 340), and calculates cash gamma l and sinh gamma l by using the product gamma l (Step 360) to obtain functions an( gamma l), bn( gamma l) and cn( gamma l). Then, absolute values delta 1, delta 2 and delta 3 of the differences between these functions and cosh gamma l or sinh gamma l are obtained, and compares the maximum delta among them with an error tolerance epsilon . If delta epsilon , the processing ends, while if delta epsilon is not satisfied, the number of division is increased by 1, and the above processing is repeated, until the number n of division for approximation is determined (Step 370). Using the determined number of division, a ladder circuit is defined, for example, by cascading PI circuits (Step 380), and stored (Step 400). IMAGE
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