The restaurant owners are suspected of arranging cheap labour for themselves in this manner over a period of several years.
The restaurants in question are foreign-owned and represent the low end of the price spectrum, says Lt. Jan Sundell of the Border Guard, who is heading the investigation.
He declined to say which countries the cheap labourers come from.
As part of the probe, the Border Guard, Kymenlaakso Police and tax authorities made joint raids on several restaurants in the Kotka and Hamina areas on Wednesday.
Last spring, they carried out searches of homes in Loviisa and Porvoo based on similar suspicions.
Preliminary investigations are likely to go on for several months.