Provided are an arithmetic method and device of a reconfigurable processor. The arithmetic device includes: an Arithmetic Logic Unit (ALU) for performing an addition and subtraction operation and a logic operation of a binary signal; a multiplier for performing a multiplication operation of the binary signal; a shifter for changing an arrangement of the binary signal; a first operand selector and a second operand selector each for selecting one of values output from the ALU, the multiplier, and the shifter; and an adder for adding the values selected by the first operand selector and the second operand selector.