Abstract: A mantissa X.sub.Mk, which satisfies 1.ltoreq.X.sub.Mk <2, is fed to a mantissa process section in order that the iterative multiplication operation of normalized floating-point numbers X.sub.k (k=0, . . . , m), written P=X.sub.0 .times.X.sub.1 .times.X.sub.2 .times.0 . . . .times.X.sub.m, is performed. In the mantissa process section, a product represented in RB (redundant binary) number representation is stored in a register, and such a product is fed back to a multiplier without being converted into a binary number. A shifter for one-place right-shift is provided between the multiplier and the register. A converter sequentially converts each RB number stored in the register into a binary number and provides to the shifter a shift-amount signal S.sub.1 whose value varies with the result of such RB-to-binary number conversion. If a digit that is positioned upward two places from the binary point of a binary number obtained by the conversion is 0, then the value of S.sub.1 is set to 0.