Suspicious script Getting injected in my website's header again and again

A suspicious script is getting injected on all my website again and again even after removing..!! Its on shared server.
Please check the screenshot :-

https://drive.google.com/file/d/0BxeA28xMdEqIaWxZYU5ObFdqZkE/view?usp=sharing

I kept on removing the the script from the header files , but not able to get the permanent solution ..

For reference please check the source code of other websites also … same code is triggered to all website..as you can see in the screenshot,

website :- https://www.airconelevators.com
oxygymfitness.in

Any help would be appreciable .!
Thanks..!!