Actuating device for a locking mechanism

The operating device has a seismic mass (3,6), moved from a rest position within a seating (1,2), in which the blocking mechanism (9) for the seatbelt reel (10) is inactive, into a blocking mechanism activation position by a braking or acceleration movement. The seismic mass is provided by a hollow ball (3) containing a solid ball (6) which fills only part of the hollow ball, the remainder filled with a gas or gas mixture.