A living tissue to be treated is brought into contact with a first high-frequency electrode (110) having high thermal conductivity. A substrate (132) on which an electric resistance pattern (134) is formed is longer than the first electrode (110). The substrate (132) includes an end portion projected from the first high-frequency electrode (110). A first lead connection portion (136) electrically connected to the electric resistance pattern (134) is formed on the projected end portion. The substrate (132) is pasted to the first electrode (110) by use of a high-thermal-conductivity heat-resistant adhesive sheet (120). A lead line for applying power to the electric resistance pattern (134) is arranged in the first lead connection portion (136) where it extends in the longitudinal direction of the first electrode (110) and on the side close to the first electrode (110).