In an isolator, a common electrode is disposed on a first surface of a magnetic plate. On a second surface of the magnetic plate, first, second, and third center conductors are disposed crossing each other. The center conductors have their respective first ends connected to the common electrode, and their respective second ends connected to matching capacitors. Furthermore, the second end of the third center conductor is connected to a terminating resistor. The matching capacitor connected to the third center conductor has a Q factor of 200 or smaller and a capacitance of 18 pF or larger. The matching capacitors connected to the first and second center conductors respectively have Q factors of 400 or larger.