To solve such problems of increasing a financial burden on an user and complexity in mounting work by using a positioner when adding a function of stopping and holding a valve in an intermediate position to a conventional single acting type pneumatic actuator carrying out two position operation, and capable of driving and holding the valve either in a full-close position or a full-open position.

In this single acting type pneumatic actuator, an additional unit 18 including a cylinder 15 for expansion, an additional piston 16 and a stopper bold 17 is mounted on an edge part 1b wherein a spring member 7 of a cylinder 1 is not arranged, thereby the function of stopping and holding the valve in the intermediate position can simply be added with a low cost.