The invention relates to a method for determining the size of a radial gap between rotating and torsion-proof parts, particularly the parts of a turbomachine. According to said method, an original signal emitted by a transmitter device located on the surface of the rotating part is received in a modified manner by a receiver device disposed on the torsion-proof part and is redirected to an evaluation unit. Said evaluation device determines and displays the size of the radial gap from the received signal by determining the parameters of the trajectory of the rotating transmitter device.