1. An elliptic curve encryption processing method using a Montgomery elliptic curve By 2 &equals;x 3 &plus;Ax&plus;x on a prime field, said elliptic curve encryption processing method comprising: in the computation of an addition point kP&plus;lQ of a point kP which is a scalar quantity k times as great as a point P on said Montgomery elliptic curve and of a point lQ which is a scalar quantity l times as great as a point Q on said Montgomery elliptic curve, an initial point computation step of calculating a set G t&plus;1 of four initial points on the basis of said point P and said point Q; a bit value detection step of detecting a combination of each bit value of k and l of binary representation data of said scalar quantities k and l; and an iterative computation process step of selecting a computation relation of a set G t of the next four points based on said set G t&plus;1 of four initial points according to a combination of each bit value of said binary representation data of k and l detected in said bit value detection step and performing a computation process of the next four points in accordance with said computation relation of said set G t, and selecting a computation relation of a set G i of the next four points based on a set G i&plus;1 of the four computed points and repeatedly performing a computation process of the next four points according to the number of bits of k or l of said binary representation data in accordance with said computation relation of said set G i.