An actuator comprising an actuator main body made up of a first housing and a second housing the second housing including a through-bore extending along its axial direction and an output piston part disposed in such a way as to be capable of sliding in said through-bore and adapted to apply a specific force to a target object by causing said output piston part to protrude from an output surface of said actuator main body, comprising: an igniter that causes powder to burn and applies driving energy for causing said output piston part to slide to said output piston part by the combustion of powder in said igniter; and a sealing member that separates a space in said actuator main body into a first space in which said igniter is disposed and a second space in which said output piston part is disposed and confines combustion products produced by said igniter in said first space, wherein said output piston part has an operative end portion that acts on said target object and a specific end portion including a specific end face that receives said driving energy, characterised in that said sealing member has a fixed end portion fixed to an inner wall that defines the space in said actuator main body, said inner wall being formed by an end face of a fitted portion of the second housing, and a contact portion that is in contact with said specific end face of said specific end portion when the powder burns in said igniter, wherein, in a state before the combustion of powder in said igniter, said contact portion is located at an initial position on an igniter side of said fixed end portion, and with the combustion of powder in said igniter, the sealing member deforms in such a way that the contact portion is turned inside out in relation to the fixed end portion, and said contact portion shifts to an operative position on an output surface side of said fixed end portion with sliding motion of said output piston part while being in contact with said specific end face. an igniter that causes powder to burn and applies driving energy for causing said output piston part to slide to said output piston part by the combustion of powder in said igniter; and a sealing member that separates a space in said actuator main body into a first space in which said igniter is disposed and a second space in which said output piston part is disposed and confines combustion products produced by said igniter in said first space, wherein said output piston part has an operative end portion that acts on said target object and a specific end portion including a specific end face that receives said driving energy, characterised in that said sealing member has a fixed end portion fixed to an inner wall that defines the space in said actuator main body, said inner wall being formed by an end face of a fitted portion of the second housing, and a contact portion that is in contact with said specific end face of said specific end portion when the powder burns in said igniter, wherein, in a state before the combustion of powder in said igniter, said contact portion is located at an initial position on an igniter side of said fixed end portion, and with the combustion of powder in said igniter, the sealing member deforms in such a way that the contact portion is turned inside out in relation to the fixed end portion, and said contact portion shifts to an operative position on an output surface side of said fixed end portion with sliding motion of said output piston part while being in contact with said specific end face.