A pre-authentication method for an Inter-RAT handover is disclosed. This pre-authentication method relates to a wireless access system, and provides a method for performing pre-authentication in case of an inter-RAT handover managed by different administrative domains. The method for performing pre-authentication before a handover in order to quickly perform an Inter-Radio Access Technology (Inter-RAT) handover, includes, transmitting a message having terminal information needed for the pre-authentication to a target authentication server, receiving security-associated information of a target network from the target authentication server, wherein the target network security-associated information is generated by the mobile terminal information, and generating security-associated information, which will be used by the mobile terminal in the target network, using the security-associated information of the target network, and performing the pre-authentication.