A treatment tool exchanging device (100, 110, 120, 130, 140) for switching and holding a plurality of treatment tools (29, 31) for treating a target to be treated, the treatment tool exchanging device (100, 110, 120, 130, 140) comprising: a flexible insertion part in which a plurality of treatment tool channels (26, 27) are formed, wherein the plurality of treatment tools (29, 31) are adapted to be inserted into the plurality of treatment tool channels (26, 27) and each treatment tool channel (26, 27) defines an insertion path for the treatment tools (29, 31) from a treatment tool port (32, 33) provided at a proximal end of the flexible insertion part to an arm (24, 25) provided at a distal end of the flexible insertion part; a driving part (42, 43) adapted to advance and retract each of the plurality of treatment tools (29, 31), within the treatment tool channel (26, 27) when they are inserted in the treatment tool channels (26, 27); a manipulating part for manipulating the plurality of treatment tools (29, 31) and treating the target; and a control unit adapted to control an amount of protrusion from the treatment tool channel (26, 27) of a treatment tool (29, 31) among the plurality of treatment tools (29, 31) in the state when the treatment tool (29, 31) protrudes from the distal end of the treatment tool channel (26, 27), wherein each of the plurality of treatment tool channels (26, 27) is elongated from the proximal end to the distal end of the insertion part, characterized in that each treatment tool channel (26, 27) divides into two branches, wherein the first branch (107, 105) of the two branches communicates with the respective distal arm (24, 25) and the second branch (104, 103) of the two branches communicates with another treatment tool channel (26, 27), wherein the insertion part has switching parts (105, 107, 121, 131, 141) for switching the insertion path of the treatment tool (29, 31), wherein each switching part is provided at a proximal end of the second branch (104, 103). a flexible insertion part in which a plurality of treatment tool channels (26, 27) are formed, wherein the plurality of treatment tools (29, 31) are adapted to be inserted into the plurality of treatment tool channels (26, 27) and each treatment tool channel (26, 27) defines an insertion path for the treatment tools (29, 31) from a treatment tool port (32, 33) provided at a proximal end of the flexible insertion part to an arm (24, 25) provided at a distal end of the flexible insertion part; a driving part (42, 43) adapted to advance and retract each of the plurality of treatment tools (29, 31), within the treatment tool channel (26, 27) when they are inserted in the treatment tool channels (26, 27); a manipulating part for manipulating the plurality of treatment tools (29, 31) and treating the target; and a control unit adapted to control an amount of protrusion from the treatment tool channel (26, 27) of a treatment tool (29, 31) among the plurality of treatment tools (29, 31) in the state when the treatment tool (29, 31) protrudes from the distal end of the treatment tool channel (26, 27), wherein each of the plurality of treatment tool channels (26, 27) is elongated from the proximal end to the distal end of the insertion part, characterized in that each treatment tool channel (26, 27) divides into two branches, wherein the first branch (107, 105) of the two branches communicates with the respective distal arm (24, 25) and the second branch (104, 103) of the two branches communicates with another treatment tool channel (26, 27), wherein the insertion part has switching parts (105, 107, 121, 131, 141) for switching the insertion path of the treatment tool (29, 31), wherein each switching part is provided at a proximal end of the second branch (104, 103).