The present invention relates to an authorization key generating method and an authorization key updating method in a mobile communication system. A terminal and a base station generate an authorization key by using a terminal random value and a base station random value that are exchanged in an authorization key generating procedure as input data. In addition, a lifetime of an authorization key is established to be shorter than a lifetime of a root key, and the authorization key is updated with an updating period that is shorter than that of the root period.