PROBLEM TO BE SOLVED: To provide interpolative algorithm for constituting an actual one dimensional (discontinuous) function from a discrete function discretely defined on a lattice point for calculating the movement while sharply keeping an interface shape of gas-liquid when making a fluid simulator. SOLUTION: This interpolative method defines a function F on an one dimensional actual area by using a function (f) and geometry of a lattice when preparing an one dimensional structural lattice constituted on the one dimensional actual area and the function (f) on the one dimensional structural lattice defined by determining a value in the center of respective cells of the one dimensional structural lattice. A point on the lattice is set to 0 on an inclination when signs of an advance difference and a retreat difference are different to the lattice for paying attention. When the signs are the same, two times of smaller one of absolute values are selected as the inclination, and a value of the function F on a partial area of the one dimensional actual area determined by the cells is determined by a linear function having a central value (f) of the cells for paying attention and having the inclination as a value in the center. COPYRIGHT: (C)2004,JPO