A system for transmission of signals between modules wherein, for example, it is possible to transmit reliably, between modules, information relating to, for example, the actuation of safety devices. A transmission module for transmitting a pulse signal of a specific period, which indicates the existence of information, generates, in time division, pulse signals by a plurality of signal generating units that are provided in parallel, and then combines these pulse signals into a single time series pulse signal. Then an attempt is made to output this pulse signal towards a receiving-side module through a relay for controlling the output, and the signals appearing at a normally-open terminal and at a normally-closed terminal of the relay are monitored to determine whether or not there is a failure, where the generation of the pulse signal is stopped when a failure is detected.