An apparatus for robot-assisted surgery as well as a method for assisting the positioning of a manipulator arm of an apparatus for robot-assisted surgery. An instrument unit is connected to a coupling unit of the manipulator arm. The instrument unit has a surgical instrument with an instrument shaft, the proximal end of which is passable through a body orifice of a patient to a target area. When connecting the instrument unit to the coupling unit the distance vector (V), which is orthogonal to the longitudinal axis of the instrument shaft of the surgical instrument, between the longitudinal axis and the target area defined by the coordinates (xz, yz, zz) is determined. A first control information is generated when the amount of the determined distance vector (V) has and/or falls below a first preset value. An output unit outputs a signal dependent on the first control information.
