Abstract: A centralized monitoring system for gas leakage in which a plurality of remote terminal units are connected by a loop line and each remote terminal units is further connected to one or a plurality of gas leakage detectors which produce analog signals representing the degree of gas concentration. Each remote terminal unit converts the analog signals from the gas leakage detectors into digitally coded block data and then transmit the block data together with an address code in time division fashion to a central monitoring station also connected to the loop line for processing the block data to provide display of the status of the gas concentration at the central station. In the transmission of the data through the loop line, a double transmission check system and a loop line switching means are employed to achieve reliable and fast data transmission.