A signal-transferring device having a first circuit and a second circuit that operate on different ground references, and a third circuit for transferring signals while providing insulation between the first circuit and the second circuit. The second circuit switches a logic level of an output signal in accordance with the logic level of an input signal notified by the first circuit, and notifies the first circuit about the logic level of the output signal. The first circuit notifies the second circuit about the logic level of the input signal not only when the logic level of the input signal has been switched, but also when the logic level of the output signal notified by the second circuit does not match the logic level of the input signal.