1. A processor system including a plurality of processors, comprising: assignment controlling means for reading instructions to be executed by the respective processors and for controlling assignments of the instructions to the processors; clock controlling means for controlling the frequencies of clock signals to be supplied to the respective processors, in accordance with the instructions to be executed by the processors according to the assignments; and voltage controlling means for controlling, in accordance with the control of the frequencies of the clock signals by the clock controlling means, at least either power supply voltages to be supplied to the respective processors or substrate voltages to be supplied to substrate nodes of transistors constituting the respective processors, wherein when the assignment controlling means makes a plurality of processors execute instructions in parallel, the clock controlling means and the voltage controlling means respectively supply clock signals each having a frequency lower than a predetermined frequency and either power supply voltages lower than a predetermined voltage or substrate voltages for providing threshold voltages higher than a predetermined threshold voltage.