A method and apparatus which enables a circuit of a small circuit scale to perform high-speed modular multiplication or modular exponentiation which are necessary in encryption or decryption in cryptic communication. To this end, modular multiplication Q=A.multidot.M mod N and modular exponentiation C=M.sup.e mod N are executed by repetition of computation of Z=U.multidot.V.multidot.R.sup.-1 mod N employing an integer R which is prime to N. The repetition of computation is executed by repeatedly operating a single circuit or by simultaneously operating a plurality of circuits of the same construction in a parallel manner.