Phil Watt’s second half header secured all three points for Belper despite them squandering the lead twice.
Watt’s looping header was cleared off the line, but the officials said the ball had cross the line as Belper secured a 3-2 win.
The Nailers took the lead just after the quarter hour mark when Alex Steadman got his first goal in his second spell with the club.
He collected Rob Stevenson’s knock down and finishing confidently on the half-volley.
The hosts equalised on the stroke of half-time through forward Luke Keen, who cut inside Watt and smashed the ball high into Danny Haystead’s goal.
Evan Garnett restored Belper’s lead five minutes into the second half with a low drilled finish past Stuart Parsons in the home goal.
Romulus got themselves back into the game for the second time on the hour mark.
Jordan Francis collected the ball on the left-wing then took on stand-in full-back Ben Watkis and slid the ball under Haystead in the Nailers’ goal.
But Phil Watt headed home the winner from a deep Jonathan Williams corner.
The Nailers could have killed the game off in the dying moments when Steadman held the ball up and back-heeled into the path of substitute Tyrell Shannon-Lewis, who somehow placed the ball wide from close range.
Both sides had chances to add to the scoreline, with the hosts’ best chance falling to goalscorer Keen.
Five minutes before the break he led a counter attack, cut inside but was brilliantly denied by Luke Shaw who blocked his effort on goal.
Liam Holt’s fizzing free-kick was impressively tipped over by Haystead to keep Belper’s lead intact.
Belper return to The Marston’s Stadium next Saturday as they look to continue their good form against Lincoln United.
Romulus: Parsons, Evans, Branch, Lee, Ezewele, Holt, Luckie, Butler (Walters 82 mins), Keen, Ballard (Steane 54 mins), Francis.
Unused subs: McDonald, Fitzharris.
Belper Town: Haystead, Shaw (Simpson 54 mins), Cole, Williams, Cooksey, Watt, Stevenson, Graves, Garnett (Shannon-Lewis 61 mins), Watkis, Steadman.
Unused subs: Mansfield, Minott, Hemagou.
Referee: Danny Gratton (Stone).