To provide a monitoring system which can cope with various modes thereof depending on the circumstances.

The monitoring system 1 is formed of a sensor device 2, a door sensor device 3, and a monitoring device 4. The sensor device 2 monitors the locking/unlocking state of a window. The door sensor device 3 detects the state of an entrance door. The monitoring device 4 performs notification of the locking/unlocking states of the monitoring objects detected by the sensor device 2 and the door sensor device 3, respectively. The monitoring device 4 is comprised of a microcomputer 43 which can change the monitoring device 4 from a security mode to an at-home mode when the state of the monitoring object is detected by the sensor device 2 or the door sensor device 3 in a predetermined mode.