A pyrotechnic actuator mechanism which has an ignition unit including an ignition charge, the pyrotechnic actuator comprising a cover member which defines a predetermined closed space for covering a release portion of a combustion product and which is arranged to transmit combustion energy of the ignition charge to a piston, and a fixing member which fixes the ignition unit before and after the combustion of the ignition charge. Then, the cover member includes a first wall member which is opposed to the release portion of the ignition unit, and a second wall member which is connected thereto and which has a free end portion embedded in the fixing member. When the ignition charge is combusted, then the cover member is moved in the propulsion direction so that the predetermined closed space is expanded in the propulsion direction of the piston, and the movement of the cover member in the propulsion direction is regulated so that a state, in which the free end portion of the second wall member is embedded in the fixing member, is maintained at a maximum propulsion position of the cover member. Accordingly, the residue, which is produced by the combustion of the ignition charge, is preferably suppressed from being discharged or emitted to the outside.
