Abstract: The invention relates to an actuator, comprising:a cylinder,one or two pistons reciprocally movable in that cylinderone or two racks, each carried by a piston,a pinion coupled to the rack(s),an output connecting part coupled to that pinion for connecting to a device for driving, such as a butterfly valve,means for connecting to a line for medium under pressure for causing the pistons simultaneously to move towards or move away from one another, thereby causing the execution of a corresponding pivoting movement of the output connecting part, andresetting spring means for forcing the pistons into a terminal rest position.The invention provides an actuator with the feature that the resetting spring means comprise at least one gas spring which is connected between the cylinder and a piston.