Are you obliged to stop at this sign in the presented situation?

Why?

The B-20 STOP sign requires you to stop before entering the priority road, but not immediately before the sign itself. Stop at the designated stopping point or, if none is marked, where you can check that you will not obstruct traffic on the priority road.

§ 21 ust. 2 rozporządzenia w sprawie znaków i sygnałów drogowych