startDrag question (why all the movie is dragged and not only the button?)

I have inside a mc (intromc ) a button that I want to be dragged.
It has been tried this :

on (press) {
startDrag (this, true);
}
on (release) {
stopDrag ();
}
It happens that all the movie clip is dragged and not only the button.
so please How to drag only the button?
thanks