The invention refers to a Locking device (200) for a pre-tensioning arrangement of a roller blind (2) or a venetian blind, comprising a sleeve (5) and a plug element (4) adapted to be inserted into the sleeve (5), a locking member (7) and a pin (6). The pin is adapted to be operated between a locking position and a releasing position. When set to the locking position the plug element (4) is prevented from being rotated in view of the sleeve (5). When set to the releasing position the plug element (4) is allowed to be rotated in view of the sleeve (5).