Abstract: In a method to measure changes of the pair of differential transmission lines, an in-phase signal is generated by combining first and second signals transmitted through the pair of differential transmission lines, a phase of the second signal being opposite to the first signal. In a transmission line diagnostic device, a signal combiner extracts the first and second signals received by a communication unit, combines the extracted those signals, and generates an in-phase signal, a detector detects the generated in-phase signal, and a determination unit determines an error when a magnitude of the detected in-phase signal is equal to or greater than a threshold value. In a liquid level detection device, a combining unit combines the first and second signals and generate an in-phase signal, a detection unit detects a voltage of the generated in-phase signal, and a calculation unit calculates a liquid level from the detected voltage.