According to Finnish reports, a number of rescue units and an army helicopter were needed to extinguish the fire at the Kymi site, based in Kuusankoski.
It was put out after a few hours, however the paper machine was temporarily shut down.
The mill has 950 staff but nobody was hurt during the incident.
The company's products include coated fine papers; UPM Finesse gloss, UPM Finesse premium silk, UPM Finesse matt, and uncoated fine paper UPM Fine. The mill produces 800,000 tonnes annually.
UPM paper machine fully functional following fire
Paper manufacturer UPM is understood to have restarted a paper machine at one of its Finnish paper mills which was forced to shut down after a fire broke out.
