An actuator according to the present invention includes a cylinder, a piston inserted into the cylinder to be free to slide, a rod that is inserted into the cylinder and connected to the piston, a rod side chamber and a piston side chamber defined by the piston within the cylinder, a tank, a first opening/closing valve provided in a first passage that connects the rod side chamber to the piston side chamber, a second opening/closing valve provided in a second passage that connects the piston side chamber to the tank, a pump that supplies a working fluid to the rod side chamber, a motor that drives the pump, an exhaust passage that connects the rod side chamber to the tank, and a passive valve that is provided in the exhaust passage and has a predetermined pressure/flow rate characteristic.