A transmission path design assistance system assisting in the design of a transmission path with different reflection specification values for each frequency is obtained. The transmission path design assistance system includes: an acquisition unit to acquire reflection specification values of a reflection characteristic of a transmission path to be designed and a constraint of characteristic impedance distribution of the transmission path; and a computation processing unit including: a reflection characteristic calculation unit to calculate the reflection characteristic from inputted characteristic impedance distribution; a reflection characteristic modification unit to modify, on the basis of the reflection specification values acquired by the acquisition unit, the reflection characteristic calculated by the reflection characteristic calculation unit; a characteristic impedance distribution calculation unit to calculate characteristic impedance distribution from the reflection characteristic modified by the reflection characteristic modification unit; and a characteristic impedance distribution modification unit to modify, on the basis of the constraint acquired by the acquisition unit, the characteristic impedance distribution calculated by the characteristic impedance distribution calculation unit and output it to the reflection characteristic calculation unit.
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