Wesley Carrington was using the most basic metal detector when 20 minutes into his first foray he found 55 gold solidus dating back more than 1,600 years.
He had begun his search in woodlands near St Albans, Herts, after watching video clips on metal detecting on the online site YouTube.
Mr Carrington said "I just thought I would give it a go. I would say after about 20 minutes it beeped. I found a coin that was gold-coloured, with a Roman figure on it."
Days later he returned to the Berkhampstead shop where he had purchased his £135 metal detector and showed them his haul.