A Montgomery inverse calculation device includes a plurality of registers each storing a value of a variable, a modulus register storing a modulus, a multiplier performing multiplication on the modulus. A comparator compares the value of the variable stored in each of the registers with an output value of the multiplier and generates a plurality of control signals. A plurality of shifters shifts bits of a value of a variable stored in a corresponding register among the registers in response to at least one first control signal, and a quotient generation block calculates a quotient of mod 2m with respect to values output from some of the shifters in response to a second control signal. A calculation block calculates an updated value of an output value of each of the shifters using the quotient in response to at least one third control signal.