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 is best remembered for being the referee at 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.
During a delay of some 15 minutes, it appeared at one stage that Kuwait's players were going to walk off the field, culminating in the president of the Kuwait FA, Sheikh al-Sabah leaving his seat in the stadium's VIP box and coming onto the pitch to talk to them.
Meanwhile, Stupar had changed his mind and disallowed Giresse's goal. The game eventually resumed with a dropped-ball at the spot where it was believed the game stopped at the whistle. A few minutes later, France scored once again, through defender Maxime Bossis. This one was allowed to stand by Stupar and France ended up winning 4-1.
After the incident a council of FIFA referees committee verbally accused Stupar in inconsistency of his decisions, yet officially the committee did not take any decisions in that regard. Stupar lost his international refereeing credentials as a result of this incident.