When forming first and second circuits on a semiconductor substrate, an isolation region is provided between the first and second circuits by embedding a conductor in the semiconductor substrate. Also, an output node of a low impedance voltage output circuit that provides a fixed voltage at low impedance is connected to the isolation region. In this way, a noise current caused by high-frequency noise generated from the first or second circuit flows into the low impedance voltage output circuit. Hence leakage of noise signals between the circuits can be suppressed.