Now I understood...
You can't declare someting as:
class BitmapTest{
var bmp:BitmapData;
...
}

This cause a compiler error.
But you can use inside constructor or method:

class BitmapTest{
function BitmapTest(){
var bmp:flash.display.BitmapData=new flash.display.Bitmap(10,10);
...
}
}
It works fine.
Thanks.

To use Bitmap class is necessary "import flash.display.bitmap". But inside my class I can't use "import" statement.
How can I create a class that access bitmap class.