The present invention relates to a method of managing a mobile multicast key using a foreign key. More specifically, the present invention relates to a method of managing a mobile multicast key using a foreign key for secure communication between a mobile terminal and a secure relay server in the region where microwaves from plural access points overlap. A method of managing a mobile multicast key using a foreign key according to the present invention has an advantage that multicast secure relay servers perform delegated authentication in advance in a region where microwaves overlap, thus reducing a delay time for authentication in a mobile terminal and it has an advantage that it can minimize an effect from changes in group keys that user's movement make, by using a primary group key and a foreign key. This results in a reduction of an overhead from update of a group key while moving, and accordingly a reduction of a delay time. In addition, it has an advantage that it centralizes functions of key management to a secure relay server, thus overcoming the limitations on processing ability or network bandwidth of a mobile terminal.