To provide a voltage controlled oscillator having a large variable width of oscillation frequency while ensuring oscillation starting performance, a P-channel MOS transistor Tr is made ON by detecting that an oscillation signal is provided with a predetermined amplitude value and oscillating operation is shifted from an initial state to a steady state by a detecting circuit OPC and a capacitor CA is connected in series with a series circuit constituted by a crystal resonator XL and a varicap diode CV. In the initial state, a load capacitance is reduced to thereby cancel an amount of reducing conductance gm of an oscillation amplifying portion to correspond to operation of the crystal resonator by a low amplitude and negative resistance necessary for maintaining excellent oscillation starting performance is provided and in the steady state, a width of changing the oscillation frequency is enlarged by enhancing an effect of the varicap diode CV.