A data processing device has an instruction decoder (1), a control logic unit (3), and ALU (4). The instruction decoder (1) decodes instruction codes of an arithmetic instruction. The control logic unit (3) detects the effective data width of operation data to be processed according to the decode result from the instruction decoder (1) and determines the number of cycles for the instruction execution corresponding to the effective data width. The ALU (4) executes the instruction with the number of cycles of the instruction execution determined by the control logic unit (3).