1. A data processing device comprising: an instruction decoder for sequentially decoding a plurality of instructions described in a program sequence and outputting a control signal respectively corresponding to the instructions, and an instruction execution unit for executing operations respectively designated by said plurality of instructions in accordance with said control signal outputted from said instruction decoder, wherein said instruction decoder decodes a first instruction among said plurality of instructions and outputs a first control signal in a first period; said instruction execution unit executes the operation designated by said first instruction in accordance with said first control signal in a second period succeeding to said first period; said instruction decoder outputs a second control signal by decoding a second instruction of which operation is executed under a predetermined condition among said plurality of instructions in a third period; and said instruction execution unit judges whether or not said predetermined condition is satisfied and executes the operation designated by said second instruction in response to a result of the judgement in a fourth period which is started after elapsing the same time as said second period or longer from the ending of said third period.