Radial gap measurement on turbines by a microwave measuring method and an evaluation of a Doppler effect which varies according to the size of the radial gap is described. At least one radar sensor embodied as a transmission and reception unit is provided in the wall of a turbine housing which is radially oriented towards the centre of the turbine. The relative speed of an outer end of a turbine blade, dependent on the size of the radial gap, is evaluated many times in relation to the radar sensor during the passage of the blade end past the same, and the course of the relative speed over time on the zero crossing constitutes a measure for the radial gap on the basis of the relation between the absolute value of the relative speed and the incline thereof in the zero crossing.