A calibration method serves for calibrating a manufacturing device for additively producing a three-dimensional object by applying layer by layer and selectively solidifying a building material. The manufacturing device comprises at least two scanning units, each of which is capable of directing a beam to different target points in the working plane, which are located within a scanning region assigned to the respective scanning unit, wherein the scanning regions region of the at least two scanning units overlap in an overlap area. At least a first of the at least two scanning units is assigned a first monitoring unit whose monitoring region extends to a target point of the first scanning unit and its proximity, wherein a change of a position of the monitoring region is carried out as a function of a change of a position of the target point.