A clock signal extractor (11) is connected to an interface of an information equipment (2) for processing an information signal, to extract a clock signal component from the information signal. A clock signal generator (12) generates a pseudo clock signal having an optional phase difference relative to the clock signal component extracted by the clock signal extractor (11). A prevention signal generator (13) generates, based on the pseudo clock signal generated by the clock signal generator (12), a leakage prevention signal having an electromagnetic wave strength stronger than electromagnetic waves leaking from the information signal. A prevention signal output unit (14) outputs the leakage prevention signal generated by the prevention signal generator (13).