Director-general Thosapone Dansuputra said the probe covered tourism, property and hotel businesses in Thailand's key tourism provinces of Phuket, Chiang Mai, Krabi, Surat Thani, Petchaburi, Prachuab Kirikan, Bangkok and Chonburi.
Thai nominees are often used by foreigners to do business or hold stakes in companies in Thailand, thereby breaching the Foreign Business Act, BE2542 (1999).
Of the five suspected cases, two are in the tourism sector and three in property businesses. Four are in Chonburi and one is in Prachuap Khiri Khan.
The department has forwarded its findings for further investigation by the Department of Special Investigation (DSI).
Breaches of the Foreign Business Act are punished with up to three years in jail and/or a fine of Bt100,000 to Bt1 million.