An automatic brake operating device to be used on a motor chain saw provided with a clutch drum around which there is a pretensioned brake band. By use of a spring and a toggle joint the band can be switched between a free position and a braking position. The toggle joint is operated between two corresponding positions by an inertia mass for releasing and a stirrup for resetting. The mass and the stirrup are individually movable and capable of triggering the brake independent of each other.