To inexpensively configure a security system of high security by using a general-purpose communication terminal device as an operating terminal device via a public line.

The communication terminal device 110 sends a signal for user identification and an operation signal to operate a monitoring device 100 to the monitoring device 100 via a telephone line 120. The monitoring device 100 compares the received signal for user identification with a personal identification number 105 or the like held therein to determine the validity thereof, and only when confirming that the signal is valid, executes a device action depending on the sent operation signal.