To provide a security system which makes it easy to register an ID code.

The security system 1 is provided with a lock opening/closing detector 2, a portable machine 3, a fire detector 4 and a monitoring device 5. The lock opening/closing detector 2, the portable machine 3 and the fire detector 4 each transmit a first to third ID signals including respectively different ID codes to the monitoring device 5. A monitor side microcomputer provided in the monitoring device 5 registers each ID code to a plurality of channels, and a change to a vacant channel for which the ID code is not registered is also made by the monitor side microcomputer subject to the registration of the ID code with the channel.