A compact frequency multiplier can perform high-degree frequency multiplication and can be incorporated in a wireless device. The frequency multiplier includes an input terminal for inputting an input signal, a first resonant circuit resonating with the frequency &ohgr; of the input signal, a second resonant circuit and an idler circuit, which resonate with the frequency n·&ohgr; that is, n times the frequency of the input signal, a variable reactor such as a varactor diode, coupling-adjusting capacitors, a bias resistor, and an output terminal for outputting multiplied output signals. In one embodiment, the frequency multiplier is a current-excitation type. The first resonant circuit and the second resonant circuit, each of which is constituted of an LC series resonant circuit, are connected in series between the input terminal and the output terminal. In addition, a parallel circuit constituted of the idler circuit and the varactor diode is connected to a connection point between the first and second resonant circuits and the input terminal and the output terminal.