PURPOSE: To realize multiplication with optional accuracy and without using any complicated circuit for rounding, by supplying the rounding signal produced with an optional bit to a multiplication/addition unit circuit set at the preceding stage so that a rounding is carried out with an optional bit.

CONSTITUTION: The rounding addition is carried out in the same way as the production of a partial product when '1' of the rounding signal is added to a sum input terminal 34 led from the preceding stage of the multiplication/ addition unit circuits B23, 26 and 29 set at the preceding stage where the multiplicand data is inputted. The rounding signal is produced by the bit selected by a round signal generating circuit 45 after the rounding control signals are received from the rounding control input terminals 43 and 44. As a result, the rounding position can be optionally changed by the rounding control signal and therefore the number of valid bits of the desired result of multiplication is also changed optionally.