There is provided a semiconductor integrated circuit that hardly causes an unnecessary operation time for variable control over an operating frequency and an internal power supply voltage. A CPU specifies an operational clock signal frequency to a clock generation circuit and an internal power supply voltage to a power supply circuit. The power supply circuit comprises a voltage regulator and a determination circuit to determine a transition state to a specified internal power supply voltage. The CPU uses a first signal to notify which time point the power supply voltage variable control start to the power supply circuit. The power supply circuit returns a second signal to the CPU to notify at which time point the power supply voltage variable control terminated. Based on handshaking between the first and second signals, the power supply voltage variable control hardly causes an unnecessary operation time for variable control over the operating frequency and the internal power supply voltage for saving power consumptions.