An isolator includes a core isolator, a main substrate and a circuit-defining section. The main substrate includes a first wiring portion, a second wiring portion and a third wiring portion and has the core isolator and the circuit-defining section mounted thereon. An input port of the core isolator is connected to the first wiring portion. An output port of the core isolator is connected to the second wiring portion. A ground port of the core isolator is connected to the third wiring portion. In the circuit-defining section, a conductor pattern includes a capacitor that is connected in parallel with the core isolator via the first wiring portion and the second wiring portion, and an impedance element that is connected to at least either of the first wiring portion and the second wiring portion.