Calibration method for calibration of a laser beam horizontal trueness testing device, the laser beam horizontal trueness testing device comprising an inherent inclination compensator and a position sensitive detector for detecting an impinging position of a laser beam in dependency of the entry angle of the laser beam with regard to the horizontal, wherein the method is carried out using a calibration device, the calibration device comprising • an elongated laser housing with a longitudinal axis, containing a laser source, whereby the alignment of the laser source is such that a laser beam is emittable by the laser source at least roughly in direction of the longitudinal axis, and • an elongated support body serving for support of the laser housing, wherein • the support body comprises at least three points of support which define a support array with a defined inserted direction for insertion of the laser housing with its longitudinal axis parallel to the inserted direction and • the size and shape of the laser housing is matched to the support array in such a way that □ the laser housing is insertable into the support array in a first face in at least two different rotational positions and in a second opposing face in at least one rotational position with the longitudinal axis as axis of rotation each and □ each contact area at the points of support in direction orthogonal to the longitudinal axis is punctiform, by which in each face (F1, F2) and each rotational position of the laser housing • the laser housing rests position-stable and self-centered due to gravity in the support array and • the position of the longitudinal axis is equal, wherein the method involves • determining an impinging position of the laser beam of the laser source on the position sensitive detector with the laser housing in at least two different rotational positions of a first face and at least one rotational position of a second opposing face each, resulting in a first, second and at least a third impinging position (P1, P2, P3), and • determining calibration parameters, particularly calibration parameters representing the point of true horizontality, based on the first, second and third impinging position (P1, P2, P3). • an elongated laser housing with a longitudinal axis, containing a laser source, whereby the alignment of the laser source is such that a laser beam is emittable by the laser source at least roughly in direction of the longitudinal axis, and • an elongated support body serving for support of the laser housing, wherein • the support body comprises at least three points of support which define a support array with a defined inserted direction for insertion of the laser housing with its longitudinal axis parallel to the inserted direction and • the size and shape of the laser housing is matched to the support array in such a way that □ the laser housing is insertable into the support array in a first face in at least two different rotational positions and in a second opposing face in at least one rotational position with the longitudinal axis as axis of rotation each and □ each contact area at the points of support in direction orthogonal to the longitudinal axis is punctiform, by which in each face (F1, F2) and each rotational position of the laser housing □ the laser housing is insertable into the support array in a first face in at least two different rotational positions and in a second opposing face in at least one rotational position with the longitudinal axis as axis of rotation each and □ each contact area at the points of support in direction orthogonal to the longitudinal axis is punctiform, • the laser housing rests position-stable and self-centered due to gravity in the support array and • the position of the longitudinal axis is equal, wherein the method involves • determining an impinging position of the laser beam of the laser source on the position sensitive detector with the laser housing in at least two different rotational positions of a first face and at least one rotational position of a second opposing face each, resulting in a first, second and at least a third impinging position (P1, P2, P3), and • determining calibration parameters, particularly calibration parameters representing the point of true horizontality, based on the first, second and third impinging position (P1, P2, P3).