A device for measuring deformations of a rotor blade of a wind turbine generator system includes a position-measuring device having a first assembly and a second assembly. The first assembly is disposed so as to be movable relative to the second assembly. The position-measuring device is configured to measure a relative position between the first and second assemblies. An arm mechanically coupled to the first assembly has a first adhesive bonding surface configured to allow the arm to be permanently joined to the rotor blade. The second assembly has a second adhesive bonding surface configured to allow the second assembly to be permanently joined to the rotor blade. A first mounting element is fixed to at the second assembly and/or the arm. A second mounting element is adhesively attachable to the rotor blade. The first mounting element is detachably joinable to the second mounting element.