A level converter level-converts an oscillation output signal of a reference frequency oscillator and supplies the level-converted signal to a phase comparator of a PLL/fractional synthesizer for controlling an oscillation frequency of an RF transmission voltage-controlled oscillator. The level converter includes a self-bias type voltage amplifier which amplifies a reference frequency signal of the reference frequency oscillator. The self-bias type voltage amplifier includes a coupling capacitor, an amplifying transistor, a load and a bias element and suppresses a variation in the level of each harmonic component even though an external power supply voltage varies.