PURPOSE: To obtain a cheap and accurate micro-positioning device by introducing working fluid pressurized with the piston of a cylinder part driven by a feed screw and a motor through a spring, into the fluid chamber of a cylindrical member expanding in the longitudinal direction and having an actuating part.

CONSTITUTION: An actuator 1 is formed by fixing its one end to a fixing part 21 and providing an actuating part 3 on the other end of a tubular member 5, which is expanded and contracted by pressure of the working fluid in a fluid chamber 2. Meanwhile, a piston 9 slidingly moving in the cylinder part 8 of a cylinder main body 7 and generating working fluid pressure is provided, and pressed with a pressing member 12 through a spring 10. This pressing member 12 is connected through a frame 15, to a driving part 22 consisting of a feed screw 14 and a motor M, a deflection quantity of the spring 10 for a target quantity of displacement of the actuating part 3 is computed by a control part 16, it is compared with the value of a displacement measuring part 13 in a comparison part 20, and hence the motor M is controlled. Consequently, displacement quantity of the actuating part and resolution power can be freely set, and micro-positioning can be performed cheaply and accurately.