Provided are a modular multiplier apparatus in which a value of a long path carry (LPC) is predicted to reduce a critical path of an arithmetic operation of Montgomery modular multiplication, and a method of reducing the critical path of the arithmetic operation. The modular multiplier apparatus for obtaining a quotient and a result of an arithmetic operation of modular multiplication by using a modulus and two arbitrary constants includes: a reduction unit for obtaining a short path carry (SPC) included when a result of a modular arithmetic operation is obtained at a current stage, by using a medium calculation result; a carry predictor for predicting a long path carry (LPC) included when the result of the modular arithmetic operation is obtained at the current stage, by using the medium calculation result; and an accumulator for accumulating the result of the modular arithmetic operation by using the SPC and the LPC, wherein the medium calculation result is obtained by adding a result of a modular arithmetic operation obtained at a previous stage and a partial product of the two constants obtained at the current stage.