A medical instrument according to the present invention includes an end effector that fulfills a predetermined function, a motor unit that generates power for driving the end effector, a transmission member that is connected to the end effector and the motor unit so as to transmit the power to the end effector, a tension holding portion that has a tension generation portion and a tension pulley which is connected to the tension generation portion and which comes into contact with the transmission member the tension pulley being configured to move such that tension applied to the transmission member in a predetermined magnitude, and a switch that can fix a position of the tension pulley.
