PURPOSE: To improve assembling operability while reducing the number of part items, and loosen stress concentration in the case that a stopper abus against a pressure vessel.

CONSTITUTION: A valve operator 1 has a pressure vessel 2 which is divided into a pneumatic pressure chamber 6 and a spring chamber 7 by means of a diaphragm 5, and a diaphragm plate arranged between a spring 9 arranged in the spring chamber 7 and the diaphragm 5. Reception parts 31 for the spring and stoppers 32 are integrally projected from the diaphragm plate. The reception parts 31 for the spring position the spring 9 for preventing the motion. The stoppers 32 are formed on an outer periphery of the diaphragm plate, abutting against an upper case 2B for limiting an upper limit of he valve opening.