PURPOSE: To obtain a root having a user's desired number of effective digits in a shorter time than before.

CONSTITUTION: The method and the device for deriving a root are disclosed by repeating a series of processings constituted of a processing for shifting values of each digit in the right direction or the left direction in registers RA, RB, RC, RD and RZ for storing a numerical value by a decimal number, based on three values of a coefficient A of a quadratic term, a coefficient B of a linear term, and a constant term C of a quadratic equation in order to derive one of two roots of the quadratic equation, simple addition and subtraction using values stored in these registers, and a simple decision based on a result of such addition and subtraction.