A processor comprises a clock signal generator generating clock signals; an operational processing part performing data processing which is divided into a plurality of execution units, in accordance with the clock signals; a storage storing data used when each execution unit is executed by the operational processing part; a data amount detector detecting amounts of the data stored in the storage per each execution unit; a clock frequency determining part determining a new clock frequency of the clock signals by using the amounts of the data, said clock signals being supplied newly to the operational processing part.