A treatment device (1, 200, 300, 400, 500) for electrosurgery comprising: a sheath which has a distal end portion and a proximal end portion; a treatment portion which has a distal end and a proximal end, is inserted into the sheath such that a direction toward the distal end from the proximal end becomes a direction toward the distal end portion from the proximal end portion, and treats a target site in at least a portion of the distal end; a first electrode (51, 251, 551) which is provided to the distal end of the treatment portion and exposed to the outside; a second electrode (52, 252, 352, 552) of which the relative positional relationship with respect to the first electrode (51, 251, 551) is fixed at a position separated from the first electrode (51, 251, 551) toward the proximal end in the treatment portion; an insulator (53, 253, 353, 553) which insulates the first electrode (51, 251, 551) from the second electrode (52, 252, 352, 552) by being interposed between the first and second electrodes (51, 251, 551; 52, 252, 352, 552); a first conductive portion (54, 55, 255, 355, 515) which applies a high-frequency current to the first electrode (51, 251, 551); and a second conductive portion (12, 13, 512) which is fixed to the distal end portion and which applies a high-frequency current to the second electrode (52, 252, 352, 552), characterized in that the second electrode (52, 252, 352, 552) is formed in cylindrical shape and is inserted into the second conductive portion (12, 13, 512) so as to be movable relative to the second conductive portion in the direction towards the distal end from the proximal end and in the direction towards the proximal end from the distal end, and a surface area of the second electrode (52, 252, 352, 552) is larger than that of the first electrode (51, 251, 551). a sheath which has a distal end portion and a proximal end portion; a treatment portion which has a distal end and a proximal end, is inserted into the sheath such that a direction toward the distal end from the proximal end becomes a direction toward the distal end portion from the proximal end portion, and treats a target site in at least a portion of the distal end; a first electrode (51, 251, 551) which is provided to the distal end of the treatment portion and exposed to the outside; a second electrode (52, 252, 352, 552) of which the relative positional relationship with respect to the first electrode (51, 251, 551) is fixed at a position separated from the first electrode (51, 251, 551) toward the proximal end in the treatment portion; an insulator (53, 253, 353, 553) which insulates the first electrode (51, 251, 551) from the second electrode (52, 252, 352, 552) by being interposed between the first and second electrodes (51, 251, 551; 52, 252, 352, 552); a first conductive portion (54, 55, 255, 355, 515) which applies a high-frequency current to the first electrode (51, 251, 551); and a second conductive portion (12, 13, 512) which is fixed to the distal end portion and which applies a high-frequency current to the second electrode (52, 252, 352, 552), characterized in that the second electrode (52, 252, 352, 552) is formed in cylindrical shape and is inserted into the second conductive portion (12, 13, 512) so as to be movable relative to the second conductive portion in the direction towards the distal end from the proximal end and in the direction towards the proximal end from the distal end, and a surface area of the second electrode (52, 252, 352, 552) is larger than that of the first electrode (51, 251, 551).