1. A code conversion method of converting a program code having an instruction operand stated therein with an assembly language code represented by a register address in a data processing system, the method comprising: an interlock detecting step of detecting an interlocked instruction among floating-point instructions included in the program code, on which the number of instructions counted until a register having the operand assigned thereto is referred to by another instruction is smaller than the number of execute cycles of the floating-point instruction; an address renaming step of changing a register address according to which an operand is referred to and assigned by the interlocked instructions to a register address not assigned as an instruction operand when the assembly language code is compiled; and a code reordering step of detecting an instruction which is not in any register-dependency relation with all the instructions between itself and the interlocked instruction among the instructions ranging from the interlocked instruction to a first label statement or branch instruction found after the interlocked instruction, and moving the detected instruction code to any position between the interlocked instructions and an instruction referring to a register in which the operand has been assigned under the interlocked instruction.