In almost all free Wi-Fi services, so that they can be used by many people, there is no encryption, such as WPA, between the access points and the devices.
If the URL of the site you are looking at begins with https (SSL communication), then communications with the site are encrypted, but be careful if you are browsing http sites or using file-sharing software or the like.

Also, it is important not to connect to any suspicious Wi-Fi.

For general information on Wi-Fi usage, refer to the “Simple Manual for Wi-Fi Users” distributed by the Ministry of Internal Affairs and Communications.

http://www.soumu.go.jp/main_sosiki/joho_tsusin/security/cmn/wi-fi/Wi-Fi_manual_for_Users.pdf