A slave cylinder (S/C) comprising: a cylinder housing a first piston to be moved forward by a motor and a second piston disposed in front of the first piston, wherein the first piston and the second piston have a first slave hydraulic chamber formed therebetween, and the first slave hydraulic chamber has a first input port which is connected to a master cylinder (M/C) and is capable of being cut off from the master cylinder by a shut-off valve, wherein the second piston and an end of the cylinder have a second slave hydraulic chamber formed therebetween, and the second slave hydraulic chamber has a second input port which is connected to the master cylinder and is capable of being cut off from the master cylinder by a shut-off valve, wherein the slave cylinder (S/C) comprises: a first atmosphere port formed to a first back chamber which is disposed in rear of the first slave hydraulic chamber and which moves together with the first piston; and a restricting means to restrict the second piston to be moved backward, characterized in that the slave cylinder further comprises a second atmosphere port formed to a second back chamber which is disposed in front of the first slave hydraulic chamber and in rear of the second slave hydraulic chamber and which moves together with the second piston, wherein the second atmosphere port is disposed in the vicinity of the restricting means in an axial direction of the cylinder. a cylinder housing a first piston to be moved forward by a motor and a second piston disposed in front of the first piston, wherein the first piston and the second piston have a first slave hydraulic chamber formed therebetween, and the first slave hydraulic chamber has a first input port which is connected to a master cylinder (M/C) and is capable of being cut off from the master cylinder by a shut-off valve, wherein the second piston and an end of the cylinder have a second slave hydraulic chamber formed therebetween, and the second slave hydraulic chamber has a second input port which is connected to the master cylinder and is capable of being cut off from the master cylinder by a shut-off valve, wherein the slave cylinder (S/C) comprises: a first atmosphere port formed to a first back chamber which is disposed in rear of the first slave hydraulic chamber and which moves together with the first piston; and a restricting means to restrict the second piston to be moved backward, characterized in that the slave cylinder further comprises a second atmosphere port formed to a second back chamber which is disposed in front of the first slave hydraulic chamber and in rear of the second slave hydraulic chamber and which moves together with the second piston, wherein the second atmosphere port is disposed in the vicinity of the restricting means in an axial direction of the cylinder. a first atmosphere port formed to a first back chamber which is disposed in rear of the first slave hydraulic chamber and which moves together with the first piston; and a restricting means to restrict the second piston to be moved backward, characterized in that the slave cylinder further comprises a second atmosphere port formed to a second back chamber which is disposed in front of the first slave hydraulic chamber and in rear of the second slave hydraulic chamber and which moves together with the second piston, wherein the second atmosphere port is disposed in the vicinity of the restricting means in an axial direction of the cylinder.