A monitoring system comprising a terminal device having terminal identifying information for identifying itself and a main control device controlling said terminal device, wherein said terminal device includes identifying-information output means that transmits the terminal identifying information of the terminal device as a radio signal in response to receipt of a request signal for the terminal identifying information wirelessly transmitted from said main control device, and said identifying-information output means is configured to send back said terminal identifying information only if main-control-device identifying information for identifying said main control device is not registered with said terminal device, said main control device includes: transmitting means that transmits said request signal for the terminal identifying information as a radio signal when a command to register the terminal identifying information is issued by user's operation; receiving means that receives the terminal identifying information as a radio signal transmitted back from said terminal device; and first storage means that stores the terminal identifying information for registering said terminal identifying information with said main control device in response to receipt of the terminal identifying information by said receiving means, wherein said transmitting means transmits said main-control-device identifying information as a radio signal to said terminal device in response to receipt of said terminal identifying information by said receiving means, and said terminal device includes second storage means that stores the main-control-device identifying information transmitted by said transmitting means for registering said main-control-device identifying information with said terminal device. said terminal device includes identifying-information output means that transmits the terminal identifying information of the terminal device as a radio signal in response to receipt of a request signal for the terminal identifying information wirelessly transmitted from said main control device, and said identifying-information output means is configured to send back said terminal identifying information only if main-control-device identifying information for identifying said main control device is not registered with said terminal device, said main control device includes: transmitting means that transmits said request signal for the terminal identifying information as a radio signal when a command to register the terminal identifying information is issued by user's operation; receiving means that receives the terminal identifying information as a radio signal transmitted back from said terminal device; and first storage means that stores the terminal identifying information for registering said terminal identifying information with said main control device in response to receipt of the terminal identifying information by said receiving means, transmitting means that transmits said request signal for the terminal identifying information as a radio signal when a command to register the terminal identifying information is issued by user's operation; receiving means that receives the terminal identifying information as a radio signal transmitted back from said terminal device; and first storage means that stores the terminal identifying information for registering said terminal identifying information with said main control device in response to receipt of the terminal identifying information by said receiving means, said transmitting means transmits said main-control-device identifying information as a radio signal to said terminal device in response to receipt of said terminal identifying information by said receiving means, and said terminal device includes second storage means that stores the main-control-device identifying information transmitted by said transmitting means for registering said main-control-device identifying information with said terminal device.