PURPOSE: To actuate a piston precisely and strongly and make its size substantially smaller, by driving the piston for extruding fluid by piezoelectric elements.

CONSTITUTION: When a voltage imposed on a third piezoelectric element 9 is stopped, it contracts for lowering a vertically moving ring 7 a little. As a result, a piston 2 held by the vertically moving ring 7 is moved downwardly by a small amount corresponding to the displacement of the piezoelectric element 9. When a voltage is imposed on a first piezoelectric element 6 and then a voltage imposed on a second piezoelectric element 8 is stopped, the piston 2 is held by a cylinder 1 and released from the vertically moving ring 7. When the voltage is imposed on the third piezoelectric element 9 under the condition, the piston 2 remains stopped and only the vertically moving ring 7 returns to an upper dead point. The voltage is supplied to the second piezoelectric element 8 again at the position and the voltage imposed on the first piezoelectric element 6 is stopped for terminating a cycle.