PURPOSE: To receive the offer of the service by a same procedure without notifying an extension and an external line by utilizing a public network additional service and an extension service from a base station and a mobile station by same operating method.

CONSTITUTION: An additional service processing section 2 offers an extension service to mobile stations 5, 6 connecting to an extension by the same procedure as that of the public network additional service and makes a request of the public network additional service to an ISDN public network by using the procedure of the public network additional service. Moreover, whether or not the service requested by the mobile stations 5, 6 is for the mobile stations 5, 6, or for an external line is discriminated by discriminating a number in the message in the procedure of the additional service or whether a message enabling the start of the additional service by the mobile station comes from the ISDN public network or from a base station 1 is discriminated and when the service is for an extension, the extension service is offered and when the service is for the external line, the ISDN public additional service is requested.