After a rock anchor (1), in particular an expandable friction tube anchor (1), comprising an anchor bolt (2) has been inserted into a pre-sunk borehole with an end sleeve (4) first, an adapter (6) having a liquid inflow and outflow is arranged on a sleeve (3). On the one hand, a holding pressure which secures the adapter (6) to the sleeve (3) is produced via a holding-pressure line (8). On the other hand, fluid flows into the anchor bolt (2) via an expansion line (7) and starts to fill said bolt. In the process, the increase in pressure (inflation pressure) causes the anchor bolt (2) to expand in the borehole, with the result that its outer surface presses against the borehole wall and thus secures the rock anchor (1) in the borehole. A gas supply connected to the adapter (6) makes it possible for a gas under pressure, in particular compressed air, to be introduced into the anchor bolt (2) before and/or after the expansion thereof. The compressed gas is relieved following the removal of the adapter (6) (pressure equalization) and conveys the fluid contained in the anchor bolt (2) virtually completely from the anchor (1).