A logic circuit system with power consumption that is reduced by automatically varying the clock frequency and operating voltage according to processing capability imposed on programmable logic circuits. The programmable logic circuits are capable of achieving plural circuit functions dynamically and can change realized circuit functions during operation. In addition, the system has a voltage supply portion for supplying a voltage to the programmable logic circuits, a clock signal supply portion for supplying a clock signal to the programmable logic circuits, a change control portion for changing the circuit functions realized by the programmable logic circuits to any one of the circuit functions, an operation time measuring portion for measuring the operation times of the programmable logic circuits to perform processing to achieve the circuit functions, respectively, and a clock-and-voltage determination portion for determining the frequency of the clock signal and the voltage, using the operation times.