A cylinder device with a force multiplier, which is configured so as to carry out lock driving downward with respect to an output rod that is inserted in a housing so as to be movable vertically, characterised by: a first piston which is inserted in a lower part of a cylinder hole formed in the housing and which is connected to a bottom of the output rod; a second piston which is inserted in an upper part of the cylinder hole and which is fitted on the output rod so as to be movable vertically; a lock chamber, provided between the first piston and the second piston, into which a pressurized fluid is supplied or from which the pressurized fluid is discharged; a first release chamber, provided below the first piston, into which a pressurized fluid is supplied or from which the pressurized fluid is discharged; a second release chamber, provided above the second piston, into which a pressurized fluid is supplied or from which the pressurized fluid is discharged; a force multiplier provided in the second release chamber so that (i) a force, which causes the pressurized fluid supplied into the lock chamber to push the second piston upward, is converted into a downward force while being subjected to force multiplication and (ii) the downward force thus converted is transmitted to the output rod, the force multiplier including (a) a force-multiplying portion provided on the second piston and (b) a plurality of engaging balls which are provided in a circumferential direction at predetermined intervals and with respect to which the force-multiplying portion carries out wedged-driving inward in a radial direction; and a retaining spring being provided in the lock chamber between the first piston and the second piston and biasing the second piston upward. a first piston which is inserted in a lower part of a cylinder hole formed in the housing and which is connected to a bottom of the output rod; a second piston which is inserted in an upper part of the cylinder hole and which is fitted on the output rod so as to be movable vertically; a lock chamber, provided between the first piston and the second piston, into which a pressurized fluid is supplied or from which the pressurized fluid is discharged; a first release chamber, provided below the first piston, into which a pressurized fluid is supplied or from which the pressurized fluid is discharged; a second release chamber, provided above the second piston, into which a pressurized fluid is supplied or from which the pressurized fluid is discharged; a force multiplier provided in the second release chamber so that (i) a force, which causes the pressurized fluid supplied into the lock chamber to push the second piston upward, is converted into a downward force while being subjected to force multiplication and (ii) the downward force thus converted is transmitted to the output rod, the force multiplier including (a) a force-multiplying portion provided on the second piston and (b) a plurality of engaging balls which are provided in a circumferential direction at predetermined intervals and with respect to which the force-multiplying portion carries out wedged-driving inward in a radial direction; and a retaining spring being provided in the lock chamber between the first piston and the second piston and biasing the second piston upward.