Hello Ano,

If you clear JEE Mains exam then it is not mandatory that you have to take JEE Advance as well.It completely depends on your choice whether you want to sit for JEE Advance or not. 

In order to give JEE Advance next year,you have to qualify JEE Mains next year as well.

In my opinion if you clear JEE Mains this year then you should sit for JEE Advance too even if you feel that you're not fully prepared. This will only add to your experience which will definitely help you next year.

Good luck!