A piezo-actuator's displacement magnifying mechanism includes a first cylinder and a second cylinder which is smaller in diameter than the first cylinder and communicates with the same. A plunger is slidably mounted in the first cylinder. The plunger is moved by a piezo-actuator in the first cylinder when a predetermined voltage is applied to the piezo-actuator to cause the same to expand and contract. A piston is slidably mounted in the second cylinder. A reservoir is communicating with the first cylinder. A hydraulic fluid is received in the first and the second cylinder and the reservoir. The hydraulic fluid is variable in viscosity in response to electric fields to which the fluid is subjected. A fluid passage is provided between the first cylinder and the reservoir for compensating for thermal expansion/contraction of the fluid. An electrode is provided for subjecting the hydraulic fluid to electric fields in the passage according to an instruction for applying the voltage to the piezo-actuator.