As a referee he officiated at about 150 matches of the Soviet Top League. Stupar twice refereed Soviet Cup finals (1979, 1981). He appeared 7 times in the Soviet annual best referees list.
He refereed the 1982 World Cup game in Valladolid, Spain, between France and Kuwait. France were leading 3-1 when their midfielder Alain Giresse broke through to score a fourth goal. The Kuwaiti players protested, claiming that they had stopped playing because they had heard a whistle from the crowd and thought it was the referee blowing. Stupar ended up disallowing Giresse's goal and the game resumed with a dropped-ball at the spot where it was believed the game stopped at the whistle. France ended up winning 4-1.