Abstract: The blocking device is intended for a cylinder lock (1) with mechanically coded tumblers and an electric coding. An installation unit (22) comprises an electrically driven control (14) with a helical drive unit (16). The helical drive unit (16) is connected with an additional blocking element (13), which can engage and disengage the rotor (10) of the cylinder lock (1). The helical drive unit (16) comprises a threaded track (39, 61, 65) closed at both ends. A control device (17) generates the control signals for the driving or the energy supply to the control (14). The method of construction according to the invention makes possible a compact type of construction and has improved security against unauthorized interference from the outside.