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 is operative upon detection of operation of mode change of the monitoring device 4 from an at-home mode to a security mode, and sets the monitoring device 4 in a security preparation mode in which the mode change from the at-home mode to the security mode can be canceled.