This goal has extended Italy's unbeaten streak to 35, equalling Spain's old European record.
Federico Keisa blasted Roberto Mancini's side into the lead from outside the box.
Atanas Iliev, who plays for Ascoli in Serie A, scored the equalizer against his team.
Italy dominated the game with 27 shots from Bulgaria's four, but Ciro Immobile's efforts, which went off the line, were too close.
They lead Group C with 10 points from four games, four points ahead of Switzerland, who have played only twice.