An authentication system according to the present invention includes an AAA server configured to be connected to the mobile network; a terminal device configured to be connected to the mobile network and be authenticated by the AAA server; and a PC configured to be connected to the terminal device through a short distance radio communication network. The PC transmits an identification data for identifying the PC itself to the terminal device through the short distance radio communication network. The terminal device authenticates the PC to access the mobile network on the basis of the identification data, and transmits an authentication data for accessing the mobile network to the AAA server. The AAA server performs an authentication that the PC accesses to the mobile network by using the authentication data. The authenticated PC accesses the mobile network through the Internet.