first of all , referring to fig5 through 8 , an applicable embodiment of the present invention includes : a main body 2 whose hollow internal part is divided into a primary pressure chamber p1 &# 39 ; and a secondary pressure chamber p2 &# 39 ;, wherein the primary pressure chamber p1 &# 39 ; can be connected with an air inlet pipe 21 and a primary pressure gauge 22 while the secondary pressure chamber p2 &# 39 ; can be connected with an air outlet pipe 23 , a secondary pressure gauge 22 and a safety decompression valve 25 , and wherein the primary and secondary pressure chamber p1 &# 39 ;, p2 &# 39 ; are respectively fitted with a through hole 26 , 27 till the top of said main body so as to form a circulation route , and wherein the through hole 26 in a through state with the narrow upper part and the wide lower part , and whereby a sealing screw 28 is disposed at the bottom end thereof while a number of screw holes 29 are mounted on the top surface of the main body 2 ; an upper cover 4 fitted with a number of screw holes 41 registering with the screw holes 29 of the main body 2 for inserting a number of screws 42 to fix the upper cover 4 on the main body 2 , and an o - shaped ring 40 is mounted on a connecting surface of the main body 2 ; and a regulator 3 whose external periphery is fitted with a piston body 32 of an o - shaped ring 31 horizontally situated inside of the upper cover 4 and which comprises a positioning body 33 at the bottom face of the piston body 32 registering with the through hole 26 of the primary pressure chamber p1 &# 39 ;, and a plug assembly 34 is penetrating through the through hole 26 upwards and a mounting rod 341 thereof is screwed in a positioning body 33 , wherein an o - ring 343 is disposed on a sealing plug 342 thereof and on the connecting face of the through hole 26 , and a return spring 35 whose both ends are properly connected to the top face of the main body 2 and the bottom face of the piston body 32 respectively so that an appropriate distance d is reserved between the bottom face of the top of the upper cover 4 and the top face of the piston body 32 when the whole body is in a free motion state , and a clearance is formed between the plug assembly 34 and the through hole 26 . the above - mentioned return spring 35 can be fixed through welding or screwing on the bottom face of the piston body 32 and the top face of the main body 2 so as to create a resilient pull for the piston body 32 . as shown in fig9 after removing the sealing screw 28 of the through hole 26 , the air inlet pipe 21 can be directly screwed at the position of the bottom face of the through hole 26 originally sealed by the sealing screw 28 without being provided with screw holes on the side face of the main body 2 . the air can then be inducted through this position . at this time , the primary pressure chamber p1 &# 39 ; is a space inside of the through hole 26 . however , this is only the change of the position of the air inlet pipe 21 , and it won &# 39 ; t be described hereinafter . furthermore , the upper cover 4 in accordance with the present invention can also be directly screwed in the thread 20 at the top of the main body 2 by means of the screw 48 at the bottom thereof . this is another applicable fixing way for the upper cover 4 in accordance with the present invention . the above - mentioned embodiment is a fixed pressure regulator . in accordance with the requirement of the special industry , in its releasing , the end of the screw can be fine - adjusted in the screw hole inside of the positioning body 33 by means of the mounting rod 341 of the plug assembly 34 so as to control the position of the plug assembly 34 in the through hole 26 when the plug assembly 34 rises and so as to reach the aim of a fixed output of the secondary pressure . this kind of the fixed air pressure regulator is designed to meet requirements of the special industry , e . g . the air pressure regulator in accordance with the present invention is applicable to the requirement of the secondary output pressure at approx . 5 kg / cm 2 without necessary adjustment in the meantime . of course , most air pressure regulators include the secondary air pressure output adjusting element to meet different pressure requirements in the work procedure . fig1 shows another embodiment of the present invention in which fine adjustment can be carried out . the middle of the upper cover 4 &# 39 ; can be shaped with a protruding form , and the inner side is formed with an accommodation 43 . the top of the upper cover 4 &# 39 ; is fitted with a screw through hole so as to install an adjusting screw 44 to elongate into the hollow room 43 in which a spring 45 is disposed while the bottom end thereof is pushed against the top of the piston body 32 . a packing piece 46 is fitted at the top end of the spring 45 for suppressing the adjusting screw 44 . surely , the adjusting screw 44 can be turned by means of a knob 47 . again , please refer to fig6 through 8 . in using the present invention , the gas flows out from the air outlet pipe 23 to lower the pressure of the secondary pressure chamber p2 &# 39 ;. at this time , the return spring 35 is in a returning free state , so that the plug assembly 34 is taken and lowered by the piston body 32 . on the other hand , the high - pressure gas passes through the air inlet pipe 21 into the primary pressure chamber p1 &# 39 ;. the primary pressure can be measured by the primary pressure gauge 22 . the high - pressure gas passes through the clearance between the plug assembly 34 and the through hole 26 into the closed room s formed by the piston body 32 and the main body 2 . thereafter , the high - pressure gas passes through the through hole 27 of the secondary pressure chamber p2 &# 39 ; of the main body 2 into the secondary pressure chamber p2 &# 39 ; and flows out through the air outlet pipe 23 for using . the secondary pressure can be measured by the secondary pressure gauge 22 . the greatest features and effects of the present invention are shown in fig6 and 7 . firstly , please refer to fig6 . when the high - pressure of the primary pressure chamber p1 &# 39 ; enters into the closed room s formed by the piston body 32 and the main body 2 and the air outlet pipe 23 creates a returning pressure , the secondary pressure inside of the closed space s pushes the piston body 32 so that the plug assembly 34 will rise . at this time , the sealing plug 342 of the plug assembly 34 and the o - ring 343 are pushed against the through hole 26 to block the passage of the primary pressure chamber p1 &# 39 ; to the closed space s in order to maintain an appropriate value of the secondary pressure . it is to note that the piston body 32 is solid and won &# 39 ; t deform in the travel process which is different from the one made of conventional elastomer piece 18 which results in deformity . moreover , the return spring 35 has a stable and balanced elasticity without the disadvantage of the bad - distributed force . furthermore , the capacity of the closed space s is much larger than that of the space of the through hole 26 in the primary pressure chamber p1 &# 39 ;. therefore , the travel change of the piston body 32 is carried out by the secondary pressure inside of the closed space s , rather than by the primary pressure effect on the plug assembly 34 to push against the piston body 32 . accordingly , the change of the pressure difference in the primary pressure chamber p1 &# 39 ; won &# 39 ; t influence the travel of the piston body 32 , so that the adjustment of the secondary pressure in accordance with the present invention is very accurate . however , the conventional air pressure regulator uses the elastomer piece 18 to push downwards against the valve assembly 17 with the negative regulating way . besides , the valve assembly 17 suppressed by the elastic pressure of the small spring 171 will be influenced by the primary pressure difference . the present invention won &# 39 ; t however be influenced by the amount of the primary pressure because 1 . the coordination effect is created by the resilience of the return spring 35 and the air pressure ; and 2 . the rise of the piston body 32 is adjusted by the positive pressure while the plug assembly 34 is synchronically moved to rise . accordingly , the stability of the output of the secondary pressure can be enhanced . the more is the pressure in the secondary pressure chamber p2 &# 39 ;, the greater is its pressure against the piston body 32 . at this time , the through hole 26 is sealed by the plug assembly 34 more firmly so that the safety will be ensured by means that the gas in the first pressure chamber p1 &# 39 ; won &# 39 ; t seep into the secondary pressure chamber p2 &# 39 ;. moreover , the conventional valve assembly 17 and the small spring 171 are easily aged , hardened etc . to result in an incomplete sealing and to cause danger . furthermore , the fine adjustment of the plug assembly 34 can be carried out to set a fixed secondary pressure output before release of the present invention . therefore , it can be directly applied to the fixed air pressure regulator to meet the requirement of the special industry . as shown in fig1 , a spring 45 and an adjusting screw 44 can be disposed for randomly regulating the output value of the secondary pressure to meet requirements . for example , the secondary pressure of the regulator 3 is preset at a certain value . it means that the regulator elongates to a preset resilience height . and the plug assembly 34 will be synchronically moved by the plug assembly 34 to close the through hole 26 . accordingly , the preset output value of the secondary pressure can be maintained . when the secondary pressure is required to increase or decrease , the knob 47 is turned to make the adjusting screw 44 to change the resilient suppressing force of the spring 45 on the top of the piston body 32 . therefore , when the original secondary pressure in the closed space s pushes against the piston body 32 , it must overcome the pressure of the spring 45 in addition to restoring the resilience of the spring 35 . accordingly , the output pressure value after adjustment can be obtained , and the greater is the resilience suppressing pressure of the spring 45 on the piston body 32 , the greater is the secondary pressure output . as for the detailed configuration of the main body 2 , the primary pressure gauge 22 , the secondary pressure gauge 24 and the safety decompression valve 25 belongs to the prior art , so that it won &# 39 ; t be described more herein . many changes and modifications in the above - described embodiment of the invention can , of course , be carried out without departing from the scope thereof . accordingly , to promote the progress in science and the useful arts , the invention is disclosed and is intended to be limited only by the scope of the appended claims .