PURPOSE: To eliminate the need for a circuit forming a special constant for the execution of rounding operation, by providing a partial reset control circuit forming a reset signal to an upper-order m-bit of a register to be the input of an operation circuit.

CONSTITUTION: An operation processing device consists of a data input terminal 1, registers 2' and 3 of n-bit length each, an operation circuit 4 inputting the registers 2' and 3, an output switching circuit 5, a data output terminal 6, a control circuit 7', and a partial reset control circuit 9 generating a partial reset signal 10. When the instruction of rounding operation of the (m+1)th or lower bits is given from the control circuit 7' to the operand in n-bit length started from the head 1st bit, the circuit 9 transmits the partial reset signal from the head 1st bit of the register 2' to the m-th bit.