1. A BCH (Bose-Chaudhuri-Hocquenghem) decoder in which a folded multiplier is equipped, the BCH decoder comprising: a key equation solver including a plurality of multipliers; wherein each of the plurality of multipliers includes a group of calculation blocks configured to perform a calculation operation, a MUX which is connected in parallel to each of the calculation blocks and supplies a first input value to the plurality of calculation blocks, a group of flip-flops connected between every two calculation blocks and configured to transfer an output value of one calculation block of the two calculation blocks as an input value of another calculation block of the two calculation blocks in synchronization with a clock signal; wherein the each of the plurality of multipliers performs a plurality of calculation stages which is folded on to the group of calculation blocks and the flip-flops connected to them, and one calculation stage is selected from the plurality of calculation stages depending on a value output from the MUX; wherein the calculation stage selected by the MUX performs the calculation operation using the group of calculation blocks and the flip-flops connected to them; and wherein the each of the plurality of multipliers outputs one output value on the basis of at least one input value in each calculation stage, and transfers an output value of a current calculation stage as an input value of the at least one another calculation block in a next calculation stage.