PROBLEM TO BE SOLVED: To provide a small actuator which achieves low costs.SOLUTION: An actuator 1 includes: a cylinder 2; a piston 3 which is slidably inserted into the cylinder 2; a rod 4 which is inserted into the cylinder 2 and is connected with the piston 3; a rod side chamber 5 and a piston side chamber 6 which are defined in the cylinder 2 by the piston 3; a tank 7; a first open/close valve 9 provided at a mid part of a first passage 8 which allows the rod side chamber 5 and the piston side chamber 6 to communicate with each other; a second open/close valve 11 provided at a mid part of a second passage 10 which allows the piston side chamber 6 and the tank 7 to communicate with each other; a pump 12 which supplies a liquid to the rod side chamber 5; and a motor 15 which drives the pump 12. In order to achieve the above object, the actuator 1 further includes a discharge passage 18 which connects the rod side chamber 5 with the tank 7; and a passive valve 19 which is provided at a mid part of the discharge passage 18 and has predetermined pressure flow rate characteristics to the liquid flowing from the rod side chamber 5 to the tank 7.