An Internet access time control method using an authentication assertion is provided. In the method, a user ID is registered in an asserting party site in a single sign-on (SSO) environment by using a user terminal. The asserting party site manages user IDs for providing an SSO service. Control setup values are set in the asserting party site by using the user terminal. Here, the control setup values include an Internet usage duration and an Internet access-restricted time band for the user. A service time of a relying party site accessed by the user terminal is controlled according to an authentication assertion containing the control setup values. Accordingly, the method makes it possible not only to control Internet usage time, but also to prevent the child from unreasonably using the Internet.