[support] problem with tmp folder and permission

[Simone Dal Maso]

Hi,
I don't know how to solve this.
In my drupal installation directory I have a folder called "tmp".
I receive an error by drupal, I read it in the log, it says approximately:
"can't write to tmp directory, because permissions are not correct". 
Sorry I'm traslating from italian.
The permissions of tmp are 755.
If I change to 777 I have no problem, but 777 is unsecure.
Is 755 a wrong number for a directory?
What should I put instead?
thanks.