Such risks can be prevented by encrypting Wi-Fi connections, but the three airports refrain from doing so in favour of user convenience, as password entry would be required for encrypted Internet connections.
Free Wi-Fi connections are available at about 900,000 locations nationwide including public facilities and convenience stores, but many of them are not encrypted, according to Masakatsu Morii, a professor at a graduate school of engineering at Kobe University.
"Users should understand there are risks and avoid exchanges of important information like credit card numbers," said Mr Morii, who conducted the research in late July.
In a test at Kansai International Airport in Osaka, Mr Morii sent e-mail and browsed the web on a PC which he monitored using a second computer running network analysis software that can be acquired free of charge. He was able to view the title, contents and delivery address of e-mail sent, in addition to the PC's IP address and URLs of websites visited.
He achieved the same results at Narita International Airport near Tokyo and Kobe airport in western Japan.
According to Mr Morii, many places like airports with a large number of visitors choose not to encrypt free Wi-Fi connections in order to skip procedures such as entering passwords.
"As we place emphasis on user convenience, we do not encrypt" Wi-Fi connections, Narita airport said, adding the service becomes available only with user consent. "If there are important communications, we encourage the use of encrypted connection."