Serial Monitor Problem - Root permissions needed

I've always used Arduino IDE on Ubuntu 12.04 without problems until today.
When I open a new Sketch everything works except the Serial Monitor: when I try to open it Arduino crashes.
I can use the Serial Monitor only when I start Arduino IDE with root permissions, and I have to type my password every time.
How can I fix it?

Thank you!

Add your user to the dialout group.

Thank you!