To provide a security system that can improve security.

The state of an object of monitoring is monitored according to a detection result of a sensor device 11. When a main power source is cut off, a backup power source 35 powers each part of a monitor device 12. Even when the main power source is cut off, the monitor device 12 can be thus operative. When a voltage detection circuit 34 detects the cutoff of the main power source, an alarm is raised to improve security. If the main power source is intentionally cut off for an intrusion into the house, for example, the alarm is raised before the backup power source 35 is disabled, which prevents the intruder from disabling the backup power source 35 to improve security.