To provide a security system capable of reducing a burden of security by reducing the generation of false alarm in the security system.

This security system is provided with a leave-from-building timer function, and furthermore, provided with a sensor 103a for guarding a security area, a timer unit 200 for counting the predetermined time when a building is set in the security condition by a security setting unit 201, and a control unit 210 for making a detecting signal, which is generated when the sensor 103a detects a human body, ineffective, in addition to making the sensor, which is provided near an entrance of the building, ineffective during the time till the time count by the timer unit 200 is completed.