A high frequency second harmonic oscillator includes a transistor, a first signal line connected at a first end to the base or gate of the transistor, a first shunt capacitor connected at a first end to a second end of the first signal line and at a second end to ground, a second signal line connected at a first end to the collector or drain of the transistor, a second shunt capacitor connected at a first end to a second end of the second signal line and at a second end to ground, and a high capacitance capacitor connected between the first signal line and the second signal line. The first signal line has a length equal to an odd integer multiple of one quarter of the wavelength of a fundamental signal, plus or minus one-sixteenth of the wavelength of the fundamental signal.