The power transmission mechanism (1) is disposed between the joint of the treatment instrument and the power generation section, and receives power from the power generation section (12A, 12B, 13). A drive member (11) for connecting the effector and the power adjustment unit and transmitting the power given from the power adjustment unit (12A, 12B, 13) to the end effector； and a power adjustment unit (12A, 12B, 13). However, as the displacement (d) of the driving member (11) increases due to the displacement of the driving member (11) caused by the bending or bending of the joint, the power transmission efficiency is increased so that the power transmission efficiency increases. Increase.動力伝達機構（１）は、処置具の関節部と動力発生部との間に配置され動力発生部から動力が与えられる動力調整部（１２Ａ，１２Ｂ，１３）と、関節部を通過してエンドエフェクタと動力調整部とを接続し、動力調整部（１２Ａ，１２Ｂ，１３）から与えられた動力をエンドエフェクタに伝達する駆動部材（１１）とを備え、動力調整部（１２Ａ，１２Ｂ，１３）が、関節部の屈曲または湾曲に伴う駆動部材（１１）の変位によって、駆動部材（１１）の変位量（ｄ）が大きい程、動力伝達効率の増加量が大きくなるように、動力伝達効率を増大させる。
