A high-frequency treatment apparatus comprising: high-frequency generating means for generating a high-frequency current; an electrode assembly adapted to be electrically connected to the high-frequency generating means, the assembly having a current-applying electrode for discharging the high-frequency current at the distal end thereof, wherein the electrode assembly has a longitudinal axis and a distal end which is branched into two segments, wherein the two segments are joined by the current-applying electrode; and an insulator that covers the two segments and proximal ends of the current-applying electrode such that a predetermined gap is formed between a inner surface of the insulator and an outer surface of the proximal ends of the electrode, wherein the electrode is bent so as to have a substantially L-shape; an insertion section adapted for receiving the electrode assembly such that the electrode assembly is movable in predetermined directions, the insertion section being located on the return side of the high-frequency current discharged from the current-applying electrode; liquid supply means (10, 12, 41, 73, 75, 75a) for supplying an irrigation liquid through the insertion section to the vicinity of the current-applying electrode; and gas supply means (70, 71, 72, 74, 74a, 77) for supplying a gas from an opening at the distal end portion of the insulator to the vicinity of the current-applying electrode through the electrode assembly; characterized in that the distal end part of the insulator is in tight contact with the current-applying electrode such that an inner duct formed in the gap between the inner surface of the insulator and the outer surface of the electrode is sealed at the distal end surface of the insulator, and a hole is formed in a periphery of the distal end portion of the insulator on a side corresponding to a direction in which the bent portion of the L-shaped electrode extends, wherein the gas is expelled through the opening along the distal end of the electrode. high-frequency generating means for generating a high-frequency current; an electrode assembly adapted to be electrically connected to the high-frequency generating means, the assembly having a current-applying electrode for discharging the high-frequency current at the distal end thereof, wherein the electrode assembly has a longitudinal axis and a distal end which is branched into two segments, wherein the two segments are joined by the current-applying electrode; and an insulator that covers the two segments and proximal ends of the current-applying electrode such that a predetermined gap is formed between a inner surface of the insulator and an outer surface of the proximal ends of the electrode, wherein the electrode is bent so as to have a substantially L-shape; an insertion section adapted for receiving the electrode assembly such that the electrode assembly is movable in predetermined directions, the insertion section being located on the return side of the high-frequency current discharged from the current-applying electrode; liquid supply means (10, 12, 41, 73, 75, 75a) for supplying an irrigation liquid through the insertion section to the vicinity of the current-applying electrode; and gas supply means (70, 71, 72, 74, 74a, 77) for supplying a gas from an opening at the distal end portion of the insulator to the vicinity of the current-applying electrode through the electrode assembly; the distal end part of the insulator is in tight contact with the current-applying electrode such that an inner duct formed in the gap between the inner surface of the insulator and the outer surface of the electrode is sealed at the distal end surface of the insulator, and a hole is formed in a periphery of the distal end portion of the insulator on a side corresponding to a direction in which the bent portion of the L-shaped electrode extends, wherein the gas is expelled through the opening along the distal end of the electrode.