An arithmetic circuit capable of Montgomery multiplication using only a one-port RAM is disclosed. In a first read process, b[i] is read from a memory M2 of a sync one-port RAM for storing a[s−1: 0] and b[s−1: 0] and stored in a register R1. In a second read process, a[j] is read from the memory M2, t[j] from a memory M1 of a sync one-port RAM for storing t[s−1: 0], b[i] from the register R1, and a value RC from a register R2, and input to a sum-of-products calculation circuit for calculating t[j]+a[j]*b[i]+RC. In a write process, the calculation result data FH is written in the register R2, and the calculation result data FL in the memory M1 as t[j]. A first subloop process for repeating the second read process, the sum-of-products calculation process and the write process is executed after the first read process.