Abstract: This invention relates to an apparatus for power measurement of laser irradiation. The apparatus according to the invention is comprised of a first elongate body forming a hollow space and consisting of a metallic wall of a small thickness. It has on a first longitudinal end thereof an opening for receiving laser irradiation. In the hollow space thereof it has at least in an area covered by the laser irradiation a black and/or rough surface. On the first longitudinal end and on the second longitudinal end thereof it respectively has a heat detector as well as a further heat detector which can be respectively coupled to a means for processing electrical output signals of the heat detectors. A second elongate body surrounds the first body on the longitudinal sides and on the second longitudinal end thereof and, together with said first body, forms an intermediate space wherein a vaporable heat transfer medium can be inserted.