A cylinder device, characterised by a force multiplier, the cylinder device comprising: a first piston inserted in a housing so as to be hermetically movable axially, the first piston having a cylindrical hole bored therethrough axially; a second piston inserted in the cylindrical hole of the first piston so as to be hermetically movable axially, the second piston having an output rod projecting outward from the housing; a lock chamber and a release chamber, formed by using the first piston and the second piston to axially compartmentalize space in the housing, into and out of which a pressurized fluid is supplied and discharged; the force multiplier, which causes (i) a first force, with which a pressurized fluid supplied into the lock chamber pushes the first piston toward the release chamber, to be transformed in a force-multiplying manner into a second force and (ii) the second force to be transmitted to the second piston, comprises engaging balls provided on an outer circumference of the second piston; and a change-over mechanism which selectively couples the first piston to the housing or the second piston via the engaging balls, the change-over mechanism receivably coupling the first piston to the housing via the engaging balls during a low-load stroke of the second piston and drivably coupling the first piston to the second piston via the engaging balls of the force multiplier during a high-load stroke of the second piston, wherein the housing has two end walls and, characterized in that the housing has a guide cylinder projecting from one of the two end walls (2a, 2b) toward the other end wall (2b, 2a), and the second piston has a guide face formed on an outer circumference thereof and inserted in the guide cylinder, the force multiplier comprising: supporting holes formed in a cylindrical wall of the guide cylinder at predetermined intervals circumferentially and bored through the cylindrical wall 6a radially; the engaging balls are received by the supporting holes, respectively; cam faces formed on the outer circumference of the second piston so that the engaging balls engage with the cam faces, respectively, each of the cam faces being formed to get closer to an axis of the second piston as it extends toward the lock chamber; and a force-multiplying face formed in the cylindrical hole of the first piston and formed to get closer to the axis as it extends toward the lock chamber. a first piston inserted in a housing so as to be hermetically movable axially, the first piston having a cylindrical hole bored therethrough axially; a second piston inserted in the cylindrical hole of the first piston so as to be hermetically movable axially, the second piston having an output rod projecting outward from the housing; a lock chamber and a release chamber, formed by using the first piston and the second piston to axially compartmentalize space in the housing, into and out of which a pressurized fluid is supplied and discharged; the force multiplier, which causes (i) a first force, with which a pressurized fluid supplied into the lock chamber pushes the first piston toward the release chamber, to be transformed in a force-multiplying manner into a second force and (ii) the second force to be transmitted to the second piston, comprises engaging balls provided on an outer circumference of the second piston; and a change-over mechanism which selectively couples the first piston to the housing or the second piston via the engaging balls, the change-over mechanism receivably coupling the first piston to the housing via the engaging balls during a low-load stroke of the second piston and drivably coupling the first piston to the second piston via the engaging balls of the force multiplier during a high-load stroke of the second piston, wherein the housing has two end walls and, characterized in that the housing has a guide cylinder projecting from one of the two end walls (2a, 2b) toward the other end wall (2b, 2a), and the second piston has a guide face formed on an outer circumference thereof and inserted in the guide cylinder, the force multiplier comprising: supporting holes formed in a cylindrical wall of the guide cylinder at predetermined intervals circumferentially and bored through the cylindrical wall 6a radially; the engaging balls are received by the supporting holes, respectively; cam faces formed on the outer circumference of the second piston so that the engaging balls engage with the cam faces, respectively, each of the cam faces being formed to get closer to an axis of the second piston as it extends toward the lock chamber; and a force-multiplying face formed in the cylindrical hole of the first piston and formed to get closer to the axis as it extends toward the lock chamber.