A first circuit outputs transmission signals that change between “H” and “L” in a period of an oscillation signal in addition to a transition time of an input signal when it changes to “H” or “L”. Control protection elements invalidate induced voltage signals obtained from transformers for first and second mask periods in response to transmission signals. Buffer circuits and Schmitt circuits generate a first signal and a second signal, each indicating “H” for a relatively long period, on the basis of “H” of the induced voltage signals. A control circuit invalidates the first signal and the second signal when both the first signal and the second signal indicate “H”.