My crank keeps undoing itself.

ro-LAND Mash, the same thing happened to me. I thought that the crank had just got loose, and retightened it. From that point on, it would loosen very quickly.

Basically I had worn down the 'square' on the BB so that needed to be replaced. The crank arm is now my office paperweight...

So, just to check. will I need a new BB, or just a new crank arm.?