Gwynedd Archaeological Trust (GAT) has been working on the site in the run-up to the start of construction of the £35m Porthmadog, Tremadog and Minffordd Bypass.
They have discovered evidence of a stone kiln and pieces of roofing tiles dating back centuries.
The trust is working closely with contractors Balfour Beatty and Jones Bros, who stripped back the topsoil along an area known locally as Bryn y Fynwent, so the experts could investigate underlying soils before construction of the new highway begins.
Partnership project manager Wyn Daniels said: “The archaeological team is very excited about the finds. It has been extremely interesting working with them and learning how key structures were constructed in the past.
“It is amazing to think that relics from the Roman era have survived so long and still exist just below our feet.”
GAT identified three distinctive areas of activity, including a deposit of roofing slate, a former stone-built kiln and patches of burning.
The team chose this part of the site to investigate after earlier archaeological work a short distance away resulted in the discovery of a former Roman bath house.
Project archaeologist, John Roberts, said: “The origin and date of these latest features are currently unclear and require further analysis, but association with a former Roman bath house has been suggested and the kiln may have been used for firing ceramic tiles.
“The shape of the slates is unusual in that they are a diamond/leaf shape, unlike modern roof slates. GAT will liaise with a slate specialist to determine the exact origin and use of these slates.”
The intention is to have the archaeological finds stored on site for public viewing within a visitors centre, details of the location and opening hours of the centre will be in the quarterly newsletter andproject website.
Pieces of Roman era pottery have also been recovered and will be investigated at a later date.
The team will continue to investigate a total of 20 different locations of the bypass site over the coming months, as part of a detailed Archaeological Mitigation Design.
The part of the route where the archaeological latest findings were unearthed is located near the junction of the A487 and the A498 to Criccieth.
GAT is one of four regional archaeological trusts in Wales, established in 1974. They maintain a range of archaeological services throughout the country. GAT employs 25 at its offices in Bangor.
The by-pass is being built for the Welsh Assembly Government under a joint venture partnership. The equal partners are Balfour Beatty and Jones Bros (Civil Engineering) UK
The long-planned new section of the A487 carriageway will improve journey times, ease congestion and significantly improve environmental conditions by enabling current through traffic to bypass the three towns of Porthmadog, Tremadog and Minffordd.