A device for measuring deformations of a rotor blade of a wind turbine generator system includes an angle measuring device and an arm. The angle measuring device includes a first and a second assembly. The first assembly is pivotable relative to the second assembly about an axis. The second assembly has a fastening site that is configured to be connected to the rotor blade. The angle measuring device is configured to measure a relative angular position between the first and the second assembly. The arm has a connection site disposed at a distance to the fastening site and is configured to be connected to the rotor blade at the connection site. The arm is mechanically coupled to the first assembly at a radial distance such that, in response to a change in the distance, a relative angular displacement is generated between the first and the second assembly.