An actuator device, comprising: a cylinder having first and second ends in its axial direction and configured to allow application of a fluid pressure of a prescribed fluid from outside to inside; a piston contained in said cylinder and slidable in accordance with said fluid pressure in said cylinder; a driving member for transmitting a reciprocating motion of said piston to the outside of said cylinder; and a first magnetic member provided outside of said cylinder over a prescribed width along a moving path of said piston; wherein said first magnetic member (112a-l) includes a plurality of coils to be independently excited, provided over said prescribed width on an outer circumference of said cylinder, said piston has a second magnetic member (202a-d) and is moved relative to said first magnetic member by excitation of said first or second magnetic member, said second magnetic member includes a plurality of permanent magnets, and soft magnetic material (203a-c) is provided between each of said plurality of permanent magnets, said permanent magnets are arranged to have opposite polarities alternately along said axial direction, and said cylinder has a first chamber defined as a space between a first inner surface at said first end of said cylinder and one side surface of said piston, and a second chamber defined as a space between a second inner surface at said second end of said cylinder and the other side surface opposite to said one surface of said piston, said actuator device further comprising: ducts (108a, 108b) for supplying said fluid pressure to each of said first and second chambers; control valves provided corresponding the ducts respectively; and a control unit for controlling said control valves and excitation of said plurality of coils; wherein said control valves are configured to control supply of said fluid pressure such that said piston is driven in the same direction as a direction of relative movement of said piston caused by excitation of said first magnetic member, and said control unit is configured to control a force generated by electromagnetic force such that a force generated by the fluid pressure is supplemented by the force generated by the electromagnetic force until the force generated by the fluid pressure reaches a desired driving force. a cylinder having first and second ends in its axial direction and configured to allow application of a fluid pressure of a prescribed fluid from outside to inside; a piston contained in said cylinder and slidable in accordance with said fluid pressure in said cylinder; a driving member for transmitting a reciprocating motion of said piston to the outside of said cylinder; and a first magnetic member provided outside of said cylinder over a prescribed width along a moving path of said piston; wherein said first magnetic member (112a-l) includes a plurality of coils to be independently excited, provided over said prescribed width on an outer circumference of said cylinder, said piston has a second magnetic member (202a-d) and is moved relative to said first magnetic member by excitation of said first or second magnetic member, said second magnetic member includes a plurality of permanent magnets, and soft magnetic material (203a-c) is provided between each of said plurality of permanent magnets, said permanent magnets are arranged to have opposite polarities alternately along said axial direction, and said cylinder has a first chamber defined as a space between a first inner surface at said first end of said cylinder and one side surface of said piston, and a second chamber defined as a space between a second inner surface at said second end of said cylinder and the other side surface opposite to said one surface of said piston, said actuator device further comprising: ducts (108a, 108b) for supplying said fluid pressure to each of said first and second chambers; control valves provided corresponding the ducts respectively; and a control unit for controlling said control valves and excitation of said plurality of coils; wherein said control valves are configured to control supply of said fluid pressure such that said piston is driven in the same direction as a direction of relative movement of said piston caused by excitation of said first magnetic member, and said control unit is configured to control a force generated by electromagnetic force such that a force generated by the fluid pressure is supplemented by the force generated by the electromagnetic force until the force generated by the fluid pressure reaches a desired driving force. ducts (108a, 108b) for supplying said fluid pressure to each of said first and second chambers; control valves provided corresponding the ducts respectively; and a control unit for controlling said control valves and excitation of said plurality of coils; wherein said control valves are configured to control supply of said fluid pressure such that said piston is driven in the same direction as a direction of relative movement of said piston caused by excitation of said first magnetic member, and said control unit is configured to control a force generated by electromagnetic force such that a force generated by the fluid pressure is supplemented by the force generated by the electromagnetic force until the force generated by the fluid pressure reaches a desired driving force.