Yu Hamaji, 34, was arrested on Sunday under an arrest warrant as he attempted to leave Thailand, immigration police said.
Police raided two houses last month in Pattaya, a popular beach resort city southeast of Bangkok, and arrested five Japanese nationals.
They included the leader of the ring, who allegedly operated a call center in Thailand targeting elderly Japanese residents.
Authorities believe Hamaji was one of the masterminds behind the operation, which swindled victims out of an estimated 300 million baht ($8.6 million).
Chaya Panakit, immigration police chief of investigation, said more than 200,000 people have been swindled by the scam.
"The maximum loss of one victim was around 100 million yen ($630,000)," he said.
The scam involved fraudsters impersonating Japanese government staff to sell bogus health insurance packages, according to the police statement.
Authorities are still searching for two suspects -- who remain at large -- with the help of the Japanese embassy in Bangkok.