A signal monitoring device, a signal transmission/reception device and a communication device, which realize easy acquisition of a diagnosis result and a signal value, and further realize fast acquisition of the diagnosis result. A signal monitoring device includes: a difference value calculator that time-sequentially calculates, for a signal to be monitored, a difference value between a signal value thereof and a previously set threshold; and a storage that updates and stores the time-sequentially calculated difference value in accordance with a previously set rule.