An So-interface for an ISDN basic access for connecting a four-wire line with a pair of receiver wires and a pair of transmitter wires to an ISDN terminal, while separating their potentials, has a capacitor (C1, C2) in each receiver wire (SR1, SR2) and a transformer (T) in each sending wire (SX1, SX2) to separate their potentials. Inductors (L1, L2, L3, L4) are connected to each receiver wire (SR1, SR2) and transmitter wire (SX1, SX2) on the interface side. The pair of receiver wires (SR1, SR2) is preferably connected on the terminal side to a voltage amplifier (OP) whose voltage amplification ratio is the same as that of the transformer (T) in the pair of transmitter wires (SX1, SX2).