A management device (181) calculates, from access information transmitted from a token terminal (121) and a site seed assigned to a server (161), a user seed, and registers the user seed in the token terminal (121). The token terminal (121) obtains a share seed to be shared with the server (161) independently therefrom, calculates a key code from the share seed and the user seed, and presents the key code to the user. When the user enters the key code to an access terminal (141), the access terminal (141) transmits, to the server (161), a request having the key code specified. The server (161) obtains access information relating to the transmitted request, calculates a checkup seed from the access information and the site seed assigned to the server (161), obtains a share seed independently from the token terminal (121), calculates a checkup code from the share seed and the checkup seed, and sets a necessary condition for sign-in that is consistent between the key code and the checkup code.