A voltage controlled oscillator includes a ring oscillator configured by connecting invertors, each of the invertors including a first and a second transistors, an operational amplifier to obtain an amplified signal, third transistors inserted between the first transistors and a first power supply, and is gate-controlled by the amplified signal, fourth transistors inserted between the second transistors and a second power supply, and is gate-controlled by the control signal, a inverter including a fifth and a sixth transistor, gate terminals and drain terminals of the fifth and sixth transistor being connected in common to a first input terminal of the operational amplifier, a seventh transistor inserted between the fifth transistor and the first power supply, and gate-controlled by the amplified signal, and an eighth transistor inserted between the sixth transistor and the second power supply, and gate-controlled by the control signal.