A semiconductor integrated circuit device includes: a switching current observer for observing a switching current; a leakage current observer for observing a leakage current; a comparator which compares the switching current and the leakage current with each other; a threshold voltage controller for controlling a substrate bias voltage in order to make a ratio of the switching current and the leakage current constant; a delay observer for observing a delay amount; and a power supply voltage controller for controlling a power supply voltage in order to keep the delay amount in a predetermined range. In the semiconductor integrated circuit device, a process which enables the minimization of an operation power is carried out by controlling the threshold voltage to make the ratio of the switching current and the leakage current constant at a given clock frequency and controlling the power supply voltage to guarantee the operating speed.