The coins were found in a huge jar just over a foot (30cm) below the surface by Dave Crisp, from Devizes in Wiltshire.
“I have made many finds over the years, but this is my first major coin hoard,” he said.
After his metal detector gave a “funny signal”, Mr Crisp says he dug down 14in before he found what had caused it.
“I put my hand in, pulled out a bit of clay and there was a little Radial, a little bronze Roman coin. Very, very small, about the size of my fingernail.”
Mr Crisp reported the find to the authorities, allowing archaeologists to excavate the site.