A pyrotechnic actuator mechanism comprises a cover member which includes a first wall member which is opposed to a release portion of an ignition unit, and a second wall member which is connected thereto and which has a free end portion embedded in a fixing member. When an ignition charge contained in the ignition unit is combusted, then the cover member is moved in the propulsion direction so that a predetermined closed space is expanded in the propulsion direction of a piston, and the movement of the cover member 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.
