1. A processor system including a plurality of processors, comprising: assignment controlling means for reading instructions to be executed by the respective processors having different functions and different performances and for controlling assignments of the instructions to the processors based on the total execution period of the instructions executed by the processor system, said different performances and an assignment information included in the instructions; 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 the clock controlling means for reducing the frequencies of clock signals supplied to the respective processors so as to reduce power consumption with predetermined execution period for the processors; 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; and instruction analyzing means for analyzing whether the instructions are executable by a plurality of processors in parallel or not, wherein the assignment controlling means makes a second executable number of processors which are more than a first executable number of processors execute instructions in parallel, the clock controlling means and the voltage controlling means respectively supply to each processor 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, and the assignment controlling means, the clock controlling means and the voltage controlling means control the assignments to the processors, the frequencies of the clock signals, and either the power supply voltages or the substrate voltages, respectively, based on a result of the analysis by the instruction analyzing means.