Abstract: An impedance converting device comprises a tri-plate structure having a first grounded conductor member (21) a second grounded conductor member (22) located apart from the first grounded conductor member to form a space between the first and the second grounded conductor members, and a line conductor member (23) located in the space for allowing a signal to pass therethrough. The impedance converting device comprises a varying section (31 to 37) for varying a terminative condition of the electric flux generated from the line conductor member. The varying section may be a window section (31 to 34) formed in the second grounded conductor member at a predetermined location. The varying section may include a metal portion (35, 36) and a slave window section (37). The window section is partially covered with the metal portion. The slave window section is connected to the window section.