To provide a security system capable of enhancing the security level of a building.

The security system includes a door sensor device 10 for detecting whether an entrance door is open or closed; a window sensor device 20 for detecting whether a window is locked or not; and a monitor device 50 having an at-home mode in which it causes the sensor devices 10, 20 to communicate with each other to monitor if the entrance door is open or closed or if the window is locked or unlocked, to keep an alarm buzzer 58 from operating, and an additional warning mode in which it operates the alarm buzzer 58 if the door is opened or if the window is locked as they are monitored. The door sensor device 10 includes an additional warning mode setting switch 12 and a door sensor microcomputer 15 that outputs a warning setting signal based upon the operation of the additional warning mode setting switch 12. The monitor device 50 has a monitor microcomputer 54 that prevents switching to the at-home mode according to the alarm setting signal.