A balanced transmission signal Tx (t) (voltage e, −e) is applied to a transmission line (L1, L2), a suppression signal (ec) is generated from a common mode suppression signal source (ec), and the suppression signal (ec) is superposed on respectives of balanced transmission signals via adders (15a, 15b). By pertinently controlling a level of the suppression signal (ec), voltage levels and phases of respectives of the transmission signals transmitted to the transmission lines (L1, L2) are controlled.