A semiconductor device which includes a frequency-variable oscillation circuit including plural inverters, each of which features a PMOS transistor and a NMOS transistor, a first substrate bias generator including a first phase/frequency compare circuit that compares an output signal from the frequency-variable oscillation circuit with a reference clock signal and generating a first substrate bias voltage in response thereto, the first substrate bias voltage being supplied to substrates of the PMOS transistors in the oscillation circuit, and a second substrate bias generator including a second phase/frequency compare circuit that compares the output signal from the frequency-variable oscillation circuit with the reference clock and generating a second substrate bias voltage in response thereto, the second substrate bias voltage being supplied to substrates of the NMOS transistors in the oscillation circuit.