The present invention relates to a safety device for portable tool with a heat engine that is capable of stopping, almost instantly, the operation of said tool after unexpected, sudden, and violent movements, said device including at least one electronic accelerometer (5) enabling the measurement of the acceleration along at least one plane or axis (X, Y, Z), the output(s) of said accelerometer (5) being connected to an electric and/or electronic monitoring means (7), wherein said electric and/or electronic monitoring means consists of an electric and/or electronic management card (7), the latter and the accelerometer being supplied with electric voltage by an electric current generator (9) known per se and actuated by the heat engine, said electric and/or electronic management card being configured to process analog or digital electrical data from the accelerometer and activate a means ensuring that said heat engine stops almost instantly.