Abstract: A security system comprising a plurality of sensors for detecting emergencies in a predetermined guard area, a transmitter-receiver for converting the emergency data into electric signals and transmitting the electric signals via a radio, a repeater for receiving the electric signals from the transmitter-receiver and transmitting the signal to a security controller, a security controller for setting a guard mode such as "start watch", and "release the watch" and transmitting the emergency and ordinary data to a central station through a transmission line such as a telephone cable or personal circuit, and a central station for supervising the predetermined guard area based on the data from this and other security controllers.