A communication system comprising: a first device being a monitoring device; and a second device being a facility management device configured to provide a predetermined service to the first device, wherein the first device comprises: a first communicator configured to communicate, in conformance with a first protocol, with the second device; and a command transmitter configured to, when the first device requests the second device to provide the service, transmit a request command to the second device via the first communicator, the request command including (i) a first header comprising parameters for receiving the service and (ii) a message body indicating a request content, and the second device comprises: a second communicator configured to communicate, in conformance with the first protocol, with the first device; a service provider (302A, 302B) configured to provide, in conformance with a second protocol, the service, the second protocol being different from the first protocol; and a relay unit configured to, upon the second communicator receiving the request command, convert, based on a predetermined message header and the first header of the request command, the request command to a frame that is in conformance with the second protocol, and supply the frame to the service provider (302A, 302B), and characterized in that the relay unit is configured to copy the predetermined message header to generate a temporary header, and to read every line in order from the first line of the first header, updates content of the temporary header based on content of the read lines, and appends the updated temporary header to the message body extracted from the request command to convert the request command to the frame. a first device being a monitoring device; and a second device being a facility management device configured to provide a predetermined service to the first device, wherein the first device comprises: a first communicator configured to communicate, in conformance with a first protocol, with the second device; and a command transmitter configured to, when the first device requests the second device to provide the service, transmit a request command to the second device via the first communicator, the request command including (i) a first header comprising parameters for receiving the service and (ii) a message body indicating a request content, and a first communicator configured to communicate, in conformance with a first protocol, with the second device; and a command transmitter configured to, when the first device requests the second device to provide the service, transmit a request command to the second device via the first communicator, the request command including (i) a first header comprising parameters for receiving the service and (ii) a message body indicating a request content, and the second device comprises: a second communicator configured to communicate, in conformance with the first protocol, with the first device; a service provider (302A, 302B) configured to provide, in conformance with a second protocol, the service, the second protocol being different from the first protocol; and a relay unit configured to, upon the second communicator receiving the request command, convert, based on a predetermined message header and the first header of the request command, the request command to a frame that is in conformance with the second protocol, and supply the frame to the service provider (302A, 302B), and a second communicator configured to communicate, in conformance with the first protocol, with the first device; a service provider (302A, 302B) configured to provide, in conformance with a second protocol, the service, the second protocol being different from the first protocol; and a relay unit configured to, upon the second communicator receiving the request command, convert, based on a predetermined message header and the first header of the request command, the request command to a frame that is in conformance with the second protocol, and supply the frame to the service provider (302A, 302B), and characterized in that the relay unit is configured to copy the predetermined message header to generate a temporary header, and to read every line in order from the first line of the first header, updates content of the temporary header based on content of the read lines, and appends the updated temporary header to the message body extracted from the request command to convert the request command to the frame.