To provide a security technique (a base station and a mobile terminal) for easily guarding a building such as a house for an average family or a small office at a low price by using a cellular phone terminal and a small base station (femtocell). A base station (FCL) of a mobile communication system placed in a building includes a communication unit (140) for communicating with the mobile communication system via a communication line; an obtaining unit (123) for obtaining an opened state of a fitting set to an opening of the building from the fitting or a switch placed near the fitting for detecting the opened state of the fitting; a registering unit (121) for registering information on a mobile terminal that uses the base station; a processing unit (122) for processing location registration of the mobile terminal; and a control unit (120) for controlling the communication unit to transmit a predetermined message to a predetermined addressee via the communication line if information received from the mobile terminal does not match the information registered in the registering unit when the location registration of the mobile terminal is performed by the processing unit after the opened state of the fitting is obtained.