1. A large-scale multiplication with addition (mul-add operation) method comprising the steps of: to perform mul-add operation of predetermined degree as recursive mul-add operation of lower-degree operation than the predetermined degree, generating a multiplication with addition operation procedure from the lowest degree to a previously defined boundary degree as a small multiplication with addition operation procedure, of a product sum procedure of recursive multiplication with addition operation from lowest-degree computation corresponding to the predetermined-degree computation; storing the generated small multiplication with addition operation procedure in procedure storage means; performing multiplication with addition operation according to the stored small multiplication with addition operation procedure, thereby generating necessary data for multiplication with addition operation from degree exceeding the boundary degree to the highest degree; generating a multiplication with addition operation procedure to the highest degree from the degree exceeding the boundary degree as a large multiplication with addition operation procedure; replacing the small multiplication with addition operation procedure stored in said procedure storage means with the generated large multiplication with addition operation procedure; and performing multiplication with addition operation according to the large multiplication with addition operation procedure stored in said procedure storage means using the generated data.