A manipulator is used to operate a surgical device that treats a body tissue. The manipulator includes a drive mechanism having a stationary section to be attached on a housing of a drive unit containing with at least one motor. A movable part is supported rotatably about a predetermined axis on the stationary section and having a connecting portion configured to be connected to the at least one motor when the stationary section is attached on the housing. A cover is configured to seal a gap between the movable part and the stationary section so as to prevent fluid from entering the gap between the movable part and the stationary part. A power transmission member is connected to both the movable part and the surgical device to transmit power from the motor to the surgical device.