To provide a cylinder which has its tool usuable as a portable unit and dispenses with complicated control members and heavy clamps therefor and which has a resilient means for returning an operation piston unit to a neutral position.

A cylinder which can be perfectly pneumatically operated has two operation pistons 26, 27 connected to an operation rod. Two small auxiliary pistons 37, 38 which are movable in an auxiliary chamber 35 are slidably attached to the operation pistons. Compressed air is successively supplied to the auxiliary chamber 35, so that the piston and rod are returned to a neutral position from the perfectly pulled right position of a rod 28 or the perfectly retracted left position thereof, by means of the pistons 37 or 38.