PURPOSE: To prevent upsizing and complicate construction of an actuator by dividing a volume chamber which is connected to a detached pressure chamber of the actuator into unit volume chambers by a shifting volume-variable means, and changing the volume of the unit volume change.

CONSTITUTION: An actuator 1 is a cylindrical elastic body and provided with pressure chambers 14, 15. Tubes 11a, 11b are provided in an extended manner from the actuator and are connected to both ends of a cylinder (a volume chamber) 21. The inside of the cylinder 21 is divided into two spaces (unit volume chambers) 21a, 21b by a piston (a volume variable means) 22. A rod 23 is provided with a driving part 24, and the shift in the axial direction is controlled by means of a control device 26. When the output of a potensiometer 25 is received by the control device 26 to shift the piston 22, the pressure difference due to the volume change of the spaces 21a, 21b allows the pressure chambers 14, 15 to be affected and the actuator 1 to be curved and expanded/contracted. This constitution prevents upsizing and complicate construction of the actuator.